Abstract: Tobacco leaves at maturing stage were inoculated with conidial suspension (1×10⁵ conidia/ml)of Alternaria alternata at different temperatures, 12, 15, 18, 21, 24, 27, 30, 33, 35, and 37℃, for various wetness durations ranging from 0 to 48 hours. Infection probability increased with the increase of wetness duration at same temperature and increased with the increase of temperature up to about 30℃, and then declined. Optimum temperature for infection was in range of 27 -30℃. Successful infection required 2 hours of leaf wetness after inoculation. A regression model using the infection probability as a function of the leaf wetness duration and temperature was established.