Microorganisms bring about most of the cellulose degradation occurring in nature. In the present study, optimization for cellulase production by Aspergillus niger was investigated by using paddy straw as substrate.Cellulase enzyme was produced by the fungus extracellularly. Optimization of cellulase production was done by using various physical (Temperature, pH, Salinity and Incubation time) and chemical parameters (Carbon sources and nitrogen sources) which could influence the enzyme activity. Cellulase production was maximum at the temperature 20°C and minimum at 40ºC. The optimal pH for the cellulase production was observed maximum in 6.0 and minimum in 7.0. Cellulase production was maximum at 48 hours and minimum at 24 hours. Cellulase production was maximum with when fructose was used as a carbon source and minimum with sucrose. Cellulase production was maximum when Malt extract was used as a nitrogen source and minimum with yeast extract. The cellulase enzyme extract was partially purified and its protein fraction was stained with its molecular weight determination by using SDS-PAGE. It revealed two protein bands with the molecular weights of about 83 kD and 50kD.