The impact of periodic drought stress on photosynthesis changes was examined in Haloxylon aphyllum and Haloxylon persicum. Parameters F 0 , Fv/Fm, the chlorophyll content and chlorophyll a/b ratio were measured. Drought stress factor was considered as lack of irrigation of potted-seedlings of two Haloxylon species in periods 0, 2, 4, 6... 26 and 28 days, and each drought period were repeated six times. Species factor included H. aphyllum and H. persicum. Drought stress significantly (p=0.01) reduced chlorophyll content and maximum quantum yield of photosynthesis (Fv/Fm ratio) but increased the Chl a/b ratio. Both the species are susceptible to drought stress.