Wheat Septoria leaf blotch causes considerable yield loss in many countries worldwide. Disease incidence depends on cultivar susceptibility, inoculum availability, management practices and favorable environmental conditions. Length of latent period is an important epidemiological factor in the subsequent spread of the pathogen within and beyond the crop. The length of the latent period depends on cultivar and environmental conditions, such as temperature and leaf wetness. Awareness of latent period provides disease forecasting in next seasons. In order to study correlation between Septoria tritici latent period and final disease severity, seven wheat cultivars and two lines were inoculated by four fungal genotypes (collected from Golestan, Ardabil and Khuzestan provinces) in the greenhouse conditions. Two-leaf seedlings of different cultivars and lines were inoculated by 2×106 spore/ml suspension and kept under mist condition for 72 hours post inoculation. After that inoculated plants were transferred to green house at 25±2ºC. Disease symptom appearance was recorded for each cultivar and line. Disease severity recorded every two days and continued till leaf death. Three replications were considered and experiment repeated twice. Relation between latent period and final disease severity of each cultivar and line was evaluated using STATGRAPHIC software. Results showed that, the best simple regression model obtained in Shanghai cultivar (S= -285.67+100.9 Log (Lp)) with 54% correlation of determination. Study of resistance and/or susceptibility of the cultivars and lines based on area under disease progress curve revealed a significant correlation between disease severity and latent period in moderate resistant cultivars but not in susceptible ones.