The role of biomass energy is becoming more and more important in renewable energy. As biomass energy utilization has the problems of collection and transportation. A small-scale dispersed power source is required. Biomass gasification CHP (combined heat and power) system converting biomass energy into gas for CHP at high temperature is one of the most effective solutions because of its high energy conversion efficiency.Dual fuel engine which can ignite low calorie biomass gasification gas by injected light oil is suitable for woody biomass gasification gas of changing calorie and amount. The effect of gas ratio which is defined as gas energy ratio in total inlet energy, injection timing, throttling and gas composition on the performance and exhaust emission of dual fuel engine was investigated by using 2 kinds of model gas which consists of H₂, CO, CH₄, CO₂ and N₂. As gas ratio of low calorie gas increases, NOx and thermal efficiency decrease but CO and THC increase. At 58% gas ratio (BMEP = 0.24 MPa) advanced injection timing can improve thermal efficiency and decrease THC and CO. When throttling was used at light load, thermal efficiency can be improved. But, because at high load thermal efficiency is highest and NOx, Sd (Smoke density) are lowest at WOT, throttle should be wide open. As first model gas has no CH₄ and high level of CO and H₂, exhaust emission of first model gas has low THC and high CO.The dual fuel engine has optimized the injection timing and has installed an oxidation catalyst to reduce CO. The dual fuel engine runs by using gas from the biomass gasification CHP system of downdraft type. The thermal efficiency is achieved over 35% and CO emission level was less than 100 ppm. Under over 70% gas ratio condition, NOx emission level is less than 300 ppm in 0% O₂.