In early stages of seedling development, heterotrophic seedling growth has been made of two portions: 1) the weight of mobilized seed reserve and 2) the conversion efficiency of mobilized seed reserve to seedling tissue. This research was carried out to evaluate germination and seedling growth of aged seeds of soybean (cv. DPX). Seed deterioration treatments were 12, 24, 36, 48, 60, 72, 96 and 120 hour at 41oC. After seed deterioration, standard germination test and seedling growth test were performed. Results showed that by seed deterioration treatments, amount of germination rate and accumulated germination percentage decrease considerably but "germination uniformity" and "germination beginning and ending time" increased. Also, seedling dry weight decreased linearly with 0.0002 mg/hour significantly when exposed to 41oC. Seed reserve utilization increased meaningfully, while conversation efficiency of mobilized seed reserve decreased considerably. It causes seedling dry weight decreased and finally seed growth power reduced.