PLTU Sinjantang in the Sawah Lunto Sumatera Barat, the PLTU produces a lot of fly ash per day 400ton/day. Fly ash can be used for combine with another material. The research is aimed to analyze of road bearing sub grade if combine with fly ash. California Bearing Ratio (CBR) and Unconfined Compressive Strength (UCS) is formula for analysis road construction. This reased wil be try haw to change properties mechanis of soil with ad fly ash.The result of gravity svecivic = 2,665 and for Natural soil, result of Unconfined Confrensive Strength (UCS), qu = 0,69 kg/cm2. If soil gived water optimum contain , the Unconfined Confrensive Strength (UCS) increansing from 0,69 kg.cm2 to 2,9 kg/cm2. It meanning of soil is Stif clay.Increasing of fly ash from 5% , 8%, 11%, 14%, 17%, and 20%. Result of Unconfined Confrenssive Strength (UCS) and unit weigth is decreassing. It means California Bearing Ratio also decreassing.