<p align="left"><em>Mulch was often applied on chili pepper cultivation. It aimed to cover the soil from various factors which can reduce the yield. This research aims to study the effect of various types of mulch (silver-black plastic and straw) to soil chemical properties, population of PSB, and chili pepper yield (harvest time, weight of dry plant biomass, and weight of fresh fruit per plant), also to study the correlation between the population of PSB with chili pepper yield. The research has been conducted from December 2015 until October 2016, on an inceptisol field in Ngemplak, Boyolali. The variables of this research are soil properties (pH, organic-carbon, total-N, available-P, and exchangeable-K), total-PSB, and chili pepper yield (harvest time, a weight of dry plant biomass, and weight of fresh fruit per plant). The result showed that various types of mulch application significantly increased soil-pH, soil total-N, soil available-P, total-PSB, a weight of dry plant biomass, a weight of fresh fruit per plant, and fastened harvest time. It also tends to increase organic-carbon and soil exchangeable-K. There were strong correlations between total-PSB and harvest time, a weight of dry plant biomass, and weight of fresh fruit per plant. The best result showed by silver-black plastic mulch application.</em></p>