The s process takes place in the stellar burning of massive stars and low mass ABG stars, which produce up to A = 90 and heavier nuclei including Pb and Bi, respectively. In theoretical studies of s process, we perform nuclear reaction network calculations for quantitative discussion on observed abundances and chemical evolution. Although astrophysical models of the s-process are well established, there is still remaining uncertainty on nuclear physics inputs. We focus on the reliability of reaction rates used in reaction network calculations, i.e., neutron-capture and beta-decay. For precise evaluation, we employ Monte-Carlo simulations with randomly varying nuclear reaction rates based on stellar evolution models.

In this presentation, we show results of Monte-Carlo based nucleosynthesis calculations focusing on the s process, based on a wide range of parameters. We cover stellar environments of massive stars from the solar-metallicity to very metal-poor stars (and rotation) as well as thermal pulses of low mass AGB stars. We found that the adopted range of estimated uncertainty for (n,g) rates, which are tens of percents, affects the production of s-process elements. Besides (n,g)-reaction, we studied impacts of beta-decay at stellar temperatures (8 - 30 KeV) on the s process. Focusing on the contribution of excited states, we clarify key important beta-decay rates that have strong impacts on branching points. We also discuss effects in multiple stellar environments, i.e., core-He burning of massive stars, fast rotating metal-poor stars and thermal pulses low mass ABG stars.