Most of stable elements (about eighty) have two and more stable (non-radioactive) isotopes. The physical properties of materials composed of such elements depend on the isotopic abundance in some extent. Remarkably strong isotope effect is observed in the phonon thermal conductivity, the principal mechanism of heat conduction in nonmetallic crystals. An isotopic disorder due to random distribution of the isotopes in the crystal lattice sites results in a rather strong phonon scattering and, consequently, in a reduction of thermal conductivity. In this paper we present new results of accurate and precise measurements of thermal conductivity κ(T) for silicon single crystals having three different isotopic compositions at temperatures T from 2.4 to 420 K. The highly enriched crystal containing 99.995% of 28 Si, which is one of the most perfect crystal ever synthesized, demonstrates the thermal conductivity about 450 W cm −1 K −1 at 24 K, the highest measured value among bulk dielectrics, which is ten times greater than one for its counterpart nat Si with the natural isotopic constitution. For highly enriched crystal 28 Si and crystal nat Si the measurements were performed for two orientations [001] and [011], a magnitude of the phonon focusing effect on thermal conductivity was determined accurately at low temperatures. The κ(T) measured in this work gives the most accurate approximation of the intrinsic thermal conductivity of single crystal silicon which is determined solely by the anharmonic phonon processes and diffusive boundary scattering over a wide temperature range.