Magnetic skyrmions are topologically protected vortex-like nanometric spin textures that have recently received growingly attention for their potential applications in future highperformance spintronic devices. Such unique mangetic naondomains have been recently discovered in bulk chiral magnetic materials, such as MnSi [1][2][3][4] , FeGe [5,6] , FeCoSi [7] , Cu 2 OSeO 3 [8][9][10] , -Mn-type Co-Zn-Mn [11] , and also GaV 4 S 8[12] a polar magnet. The crystal structure of these materials is cubic and lack of centrosymmetry, leading to the existence of Dzyaloshinskii-Moriya (DM) interactions. Unlike the conventional spin configurations, such as helical or conical, that are usually found in chiral magnets, a magnetic skyrmion has a particle-like swirling-spin configuration characterized by a topological index called the skyrmion number [13,14] . The nontrivial topology of magnetic skyrmions results in a number of

Two-dimensional (2D) van der Waals (vdW) magnetic materials have recently been introduced as a new horizon in materials science and enable the potential applications for next-generation spintronic devices. Here, in this communication, the observations of stable Bloch-type magnetic skyrmions in single crystals of 2D vdW Fe3GeTe2 (FGT) are reported by using in-situ Lorentz transmission electron microscopy (TEM). We find the ground-state magnetic stripe domains in FGT transform into skyrmion bubbles when an external magnetic field is applied perpendicularly to the (001) thin plate with temperatures below the Curie-temperature TC. Most interestingly, a hexagonal lattice of skyrmion bubbles is obtained via field cooling manipulation with magnetic field applied along the [001] direction. Owing to their topological stability, the skyrmion bubble lattices are stable to large field-cooling tilted angles and further reproduced by utilizing the micromagnetic simulations. These observations directly demonstrate that the 2D vdW FGT possesses a rich variety of topological spin textures, being of a great promise candidate for future applications in the field of spintronics.KEYWORDS: magnetic skyrmions, van der Waals materials, Fe3GeTe2, Lorentz transmission electron microscopy 3 Two-dimensional (2D) van der Waals (vdW) materials are a family of quantum materials that have attracted great research attention in the past decade as they possess a diverse range of novel phenomena which are promising for technological applications. 1,2 In particular, the recent discovery of magnetic 2D vdW materials, such as Cr2Si2Te6/Cr2Ge2Te6, 3-5 CrI3/CrBr3, 6, 7 and Fe3GeTe2 (FGT), 8, 9 not only offers exciting opportunities for exploring new physical properties, but also opens up a new way for developing spintronic devices by applying magnetism as a possible altering parameter. 10 Among these materials, FGT is only ferromagnetic metal, in which a long-range ferromagnetic order has been confirmed experimentally ranging from bulk crystals down to monolayers. [11][12][13] Remarkably, bulk crystalline FGT has the highest Curie temperature TC (∼230 K) and the TC of layered FGT can be raised to room temperature via electrostatic gating 8,14 or in patterned microstructures. 13 Following this discovery, many intriguing magnetic and transport properties, such as extremely large anomalous Hall effect, 15 Planar topological Hall effect, 16 Kondo lattice physics, 17 anisotropy magnetostriction effect, 18 and spin filtered tunneling effect, 19 have been observed experimentally in exfoliated FGT nanoflakes and its heterostructures.Moreover, 2D vdW FGT exhibits a strong out-of-plane uniaxial magnetic anisotropy down to atomic-layer thicknesses, 8,9,14,20 which is very critical for spintronic applications, typically, magnetic-tunneling-junctions and magnetic randomaccess-memory devices. On the other hand, in a magnetic material, the competition between the uniaxial magnetic anisotropy and magnetic dipole-dipole interaction, can emerge and lead to a diversity of...