Low angle laser light scattering is a useful method for the measurement of mole-cular weight of pulymer．In this work a convenient calibration method has been de-vised by using pure organic liquids of known Rayleigh factors to evaluate the instru-ment constant and the background simultaneously for a low angle laser light scattering photometer made in our laboratory．Considering the major background comes from the light scattered in the cell and the probability of stray light passing through the field stop is small，the Rayleigh factor may be rewritten asRθ/n=φ[Gθ/Go)-δ] for small scattering angles，where Go Gθ are electrical signals propotional to the radiant power in the direction θ and in the direction of incident beam respectively，n is reflective index of liquid，δ is a factor related to the background．φ is an instrument constant．A linear relationship is obta-ined from the plot of Rθ/n VS(Gθ／Gθ)of the pure liquids such as toluene，benzen，cyclo-hexane

carbon tetrachloride and chloroform used for calibration．The slope of the straight line is φ and the intercept φδ is related to background．Performance of this instrument and the calibration method has been evaluated by measuring ten poly-styrene samples of narrow MWD and good results were obtained．