Abstract: The experiment was carried out to study importance of photosynthesis capacity (NAR, CGR and LAI) during seed filling period and its relationship with grain and biological yield. Maximum grain yield of 437.2 g m-2 was attained by early planted crop (May 10). The rate of decrease in grain yield was noted about 22.5 g m-2 reduction with one week delay in sowing. Grain yield increased 38% with increase in plant density from 20 to 60 plants m-2 and maximum grain yield of 433.0 g m-2 was recorded at 60 plants m-2. Significant Relationship (r = 0.93, p<0.01) between biological yield at early seed filling stage (R5) and grain yield indicate that greater total dry matter results in greater seed yield if the total dry matter is produced before seed filling period. No NAR differences were observed among planting dates during vegetative stage and early reproductive stage (early flowering to early seed filling) and LAI during vegetative stage (emergence to early flowering) was 16.66% greater for late sowing date, therefore more dry matter accumulation at early seed filling stage (R5) for early planted soybean is commonly due to greate LAI during early reproductive period (R1 to R5). No NAR differences were observed among plant densities during vegetative stage (emergence to early flowering), therefore more dry matter accumulation at early seed filling stage (R5) for highest plant density is commonly due to greater LAI during emergence to early seed filling and greater NAR during early reproductive stage (R1 to R5). NAR (source activity) during seed filling period was a poor predictor of grain yield while, LAI (source size) at early seed filling stage (R5) strongly affected CGR, total dry accumulation and grain yield.