Susceptible and resistant tomato lines were artificially inoculated with Phytophthora infestans strain T1 during different seedling ages of 3,6,9 leaves.Results indicated that with the seedling aging the susceptible lines presented lower susceptible level.The disease index decreased 28.8% and 3.9% respectively for 6 leaves stage compared to 3 leaves stage and 9 leaves stage compared to 6 leaves stage,while the difference between them was not significant.The disease index of 9 leaves seedlings decreased 31.0% than 3 leaves ones and the difference was significant at 5% level.Regarding to the resistant lines,the disease index of 6 and 9 leaves reduced also about 50% when compared to 3 leaves seedling.The difference was significant at 1% level.The resistance of 6 and 9 leaves age seedling was almost the same.