In order to investigate the effect of planting date and maturity group of cultivars on seed health and incidence of seed borne fungi of soybean in Gorgan, a field experiment conducted with split plot arrangement in a randomized complete block design in three replications during 2011. The main plots included 5 planting dates (April 20, May 13, June 3, June 29 and July 22), and the sub plots included 3 crop cultivars (Williams, Sahar and Katool). For assessment of seed health, identification of seed borne fungi, and determination of seed infection rate in seed masses, 50 seeds from any seed mass planted on potato-dextrose agar (PDA) medium. Planting date had no significant effect on Williams seed health, but seed health linearly decreased by delays in planting of other cultivars (Sahar and katool). Seed health negatively correlated with seed infection by Alternaria sp. and Fusarium sp. in all cultivars. Also, a significant negative correlation were observed between seed health and seed infection by Cercospora sp. in katool cultivar. Phomopsis sp. were observed in early (all cultivars) or late planting dates (katool cultivar). Again, The other two fungies including Sphaeropsis sp. and Nigrospora sp. that observed for the first time in soybean seeds, were derived from early planting date in seeds of Williams cultivar. Thus, the delay in planting and using late maturity group cultivars, reduces the soybean seed health due to synchronization of seed filling period with the high air relative humidity and rainfall during this period.