This research aimsto study the effect of single and mixed infection of Potato Y Virus (PVY) and Cucumber mosaic virus(CMV) on the number of leaves, number of branches, plant height and stem circumference of tomato plants (cv.Elegro and Local). The experiment was carried out in 2012 in a greenhouse in Tartous. Results show that the interactions in both varieties to virus infections are different. The mixed infections cause slight and weak effects compared to single infections. This is probably due to the antagonistic relationship between Potato Y virus and Cucumber mosaic virus and their effects on mean number of leaves, mean number of branches, mean height of plants, and mean stem circumference. Tomato plants' stems were (Elegro 106.42, 16.75, 103.58 cm, 4.84cm.; Local 94.42,15, 87.17 cm, 4.59 cm, respectively). Consequently, the timing of mixed infection playsarole in appearance, development and effect on other viruses.