Abstract: Light energy conversion efficiency of an algal culture was determined according to the equotion of PE=KY, in which K=the calorific value of the biomass (Kcal per g dry weight), and Y=the growth yield, i.e. the amount of dry cells biomasss produced per Kcal light energy absorbed. The culture was performed in a 10 liters glass fermentor with continuous illumination, equiped with a sensor tube for measuring cell concentration in the medium. Kettering radiometer was used for light energy measurement. When cells were grown in the absence of combined nitrogen, the growth yield YN2=0.013, based on 6.44 for the calculated calorific value of the biomass, thence the maximum PEN2 was 8.1%; while the cells were grown on combined nitrogen (NH4CI) medium, the growth yield YNH4, calorific value of the biomass KNH+4 and the maximum PENH+ were 0.008, 6.74 and 5.8% respectively.