The behavior of peat soil's physical characteristics and the mechanism are extremely influenced by the decomposition degree. This study aims to know a physique range alteration and mechanism of the peat soil fibrous reviewed from the enhanced value of California Bearing Ratio (CBR) after it is stabilized using Effective Microorganism (EM). The volume of EM inserted and combined are 20% and 40% that ripened for 14 and 21 days. Then the peat soil and EM are mixed with water content close to original peat soil volume. The results showed that the most significant change in peat soil's physical characteristics and the mechanism is 40% EM solution with maximum decomposition rate occurs at the 14-days ripening period. The fiber content is reduced from 60% to 52%, and decomposition rate 0% fiber/day to 0.57% fiber/day, and the increased value of unsoaked CBR 3.28% to 7.47% and 3.01% to 6.11% in wet conditions.