A field experiment to study the competition between soybean and sesame in different planting ratios of intercropping based on replacement method was conducted based on a randomized complete block design with four replications at Sari Agricultural Science and Natural Resources University 2019. The planting ratios were 0:100, 25:75, 50:50, 75:25, and 100:0 (Soybean: Sesame respectively) using replacement method. Results showed that intercropping had a significant effect on the percentage of N derived from the atmosphere and light use efficiency. Among the different planting ratios, the maximum amount of biological nitrogen fixation was related to the 90-days after planting, and the 75:25 and 25:75 planting ratios had the highest (75.95%) and lowest activity rates (42.67), respectively. In addition, the highest light use efficiency in intercropping was related to the 120-days after planting. At this stage, the light use efficiency in 75:25 and 50:50 ratios were 2.03 and 1.92 g.MJ.-1, respectively. The highest land equivalent ratio was 50:50, with an average of 1.12. Finally, increasing the light use efficiency in planting ratios of 75:25 and 50:50 an essential in increasing soybean's biological nitrogen fixation and increasing the efficiency of intercropping cultivation.