In order to evaluate methanol effect on Radiation Use Efficiency, Photosynthetic Active Radiation and Extinction Coefficient of soybean, a field experiment done in the basis of randomized complete block design with three replication in faculty agricultural of Islamic Azad university, karaj branch in 2008. First factor was sprayed aqueous solutions 0 (control), 7, 14.21, 28 and 35% (v/v) methanol in 3 times during growth season of soybean with 12 days intervals on shoot of soybean. In this study were measured: grain yield, dry matter, and optimum leaf area, Radiation Use Efficiency, Extinction Coefficient and Crop Growth Rate. Analyses of variance results showed that foliar application of methanol was effective on Radiation Use Efficiency and there is a significant difference between applying treatments in 5% level of possibility. Mean comparing shows that, the most quantity of Radiation Use Efficiency was belonging to 21% methanol aqueous solution level that in comparing with control has an increase of 15%. As well as results show that after 96 days passing of planting, a significant difference in accumulation Photosynthetically Active Radiation by soybean bush was observed in different methanol treatments. Sprayed methanol solution do not have any effects on Extinction Coefficient, but in spite of this, Extinction Coefficient in comparing with other level shows a decrease in level of 21%. Methanol foliar application increased seed yield, Radiation Use Efficiency, Total Dry Weight and Leaf Area Index.