To study the effect of glyphosate application rate and timing on johnsongrass (Sorghum halepense) vegetative and reproductive growth, three experiments were conducted in field, greenhouse, and laboratory in the Agronomy Department of University of Tehran, Karaj, in 2007 & 2008. In each experiment, treatments were arranged in completely randomized (greenhouse & laboratory) and completely randomized block design (field). The experimental factors included time of treatment and glyphosate doses in greenhouse & laboratory (0, 1, 0.5+0.5, 2, 1+1, 4, 2+2 lit/ha) and as well in the field (0, 2, 1+1, 4, 2+2, 6, 3+3, 8, 4+4 L/ha). Glyphosate toxicity was quantified by visual evaluation of injury and percent reduction in dry weight, 4 weeks following treatment. Shoots, dry weights for regrowth were evaluated in greenhouse and for field in 42 days past harvesting. Treated-seed germination characteristics in different growth stages were studied in the laboratory. The results showed: a) In greenhouse, the johnsongrass control was best when sprayed at 3-5 leaf stage with 2+2 lit/ha. b) In field, the results were similar to those in greenhouse,while for the control it was suitable at flowering stage. Split doses of glyphosate resulted in better shoot regrowth control at flowering stage than single whole doses. c) In laboratory expriments, flowering stage was the best for controlling johnsongrass reproductive growth. Maximum decrease in seed production, percent and rate of germination, as well as in seed viability were obtained at flowering stage with 2 lit/ha of glyphosate. Based on the obtained results, the control of vegetative and reproductive growth of johnsongrass was best achieved when sprayed at flowering stage.