Relativistic millicharged particles (χq) have been proposed in various extensions to the Standard Model of particle physics. We consider the scenarios where they are produced at nuclear reactor core and via interactions of cosmic-rays with the earth's atmosphere. Millicharged particles could also be candidates for dark matter, and become relativistic through acceleration by supernova explosion shock waves. The atomic ionization cross section of χq with matter are derived with the equivalent photon approximation. Smoking-gun signatures with significant enhancement in the differential cross section are identified. New limits on the mass and charge of χq are derived, using data taken with a point-contact germanium detector with 500 g mass functioning at an energy threshold of 300 eV at the Kuo-Sheng Reactor Neutrino Laboratory.Big Bang nucleosynthesis [12,13], and direct laboratory experiments [14][15][16].The theme of this article is to report new direct laboratory limits on χ q using data acquired by an n-type Point-Contact Germanium (PCGe) detector with sub-keV sensitivity and excellent energy resolution. The PCGe technology [17] is being used in the studies of coherent neutrino nucleus elastic scattering, as well as searches of "light" WIMPs [18][19][20][21] and neutrinoless double beta decay.This article is organized as follows. Three cases of χ q are selected for investigations, whose origins and flux estimations are given in Section II. The interaction of χ q with matter and the derivation of cross sections are discussed in Section III. Physics constraints on χ q derived from data taken with the PCGe at the Kuo-Sheng Reactor Neutrino Laboratory (KSNL) [17,[22][23][24][25] are presented in Section IV. II. SOURCES OF MILLICHARGED PARTICLES A. Nuclear Reactor-χqNuclear reactor cores are powerful sources of γ-rays and therefore have been used for searches of axions [26,27], dark photons [28,29], and millicharged neutrinos [30]. Approximately half of the γ-radioactivity are of "prompt" origins directly from highly excited fission frag-arXiv:1808.02719v2 [hep-ph]