Quantum mechanical scattering calculations of rotational quenching of CS (v=0) collision with He-3 are performed at ultracold temperatures and results are compared with isotopic He-4 collision. Rotational quenching cross sections and rate coefficients have been calculated in the ultracold region for rotational levels up to j=10 using the He-CS potential energy surface computed at the CCSD(T)/aug-cc-pVQZ level of theory. The quenching cross sections are found to be two orders of magnitude larger for the He-3 than the He-4 isotope under ultracold conditions. Wigner threshold law is found to be valid below 10(-3) K temperature. (C) 2016 Elsevier B.V. All rights reserved.