Cucumber ( Cucumis sativus ) is a warm season monoecious plant, in which the ratio of female to male flowers, in field grown cultivars is 1 to 7. By application of some treatments such as plant growth regulators it is possible to increase the number of female flowers and hence increase fruit yield. Two experiments were carried out at Isfahan University of Technology to investigate the effects of spermidine and methyl jasmonate on sex expression of cucumber in a randomized complete block design in two seasons. The first experiment was done in autumn 2013 and consisted of seven treatments: 0.25, 0.5 and 0.75 mM spermidine and 0.1, 0.25 and 0.5 mM methyl jasmonate and control ( distilled water ) in four replications. The second experiment carried out in spring 2014 and consisted of four treatments, 0.5, 0.75 mM spermidine and 0.5 mM methyl jasmonate and control (distilled water) in 3 replications. Cucumber plants were foliar sprayed at 2 to 4 leaf stage. In both experiments, time to flowering, first flowering node, the number of female and male flowers, fruit weight (yield), main stem length, number of lateral branches, plant fresh and dry weight, chlorophyll content, fruit color quality, the antioxidant activity and vitamin C, fruit firmness, total soluble solids of fruit were evaluated. Results of the two experiments were consistent, and showed that treatment with spermidine shortened the time to flowering, caused flowering in the lower nodes, increased the number of female flowers, reduced the number of male flowers per plant and increased plant fruit yield, number of lateral branches, number of nodes, crown diameter, plant antioxidant activity, total chlorophyll content of leaves, vitamin C of fruit, color quality of fruit and reduced the fruit total soluble solids significantly compared with control. Spermidine had no significant effect on the plant fresh and dry weight and length of the main stem. Methyl jasmonate treatment caused early flowering, and reduced the number of male flowers but showed no significant effect on first flowering node, the number of female flowers and yield compared to control. Application of methyl jasmonate also led to significant increase in number of lateral branches, plant height and leaf chlorophyll content but reduced plant dry weight, while had no significant effect on crown diameter, number of nodes, fruit vitamin C, fruit soluble solids and color of the fruits compared to the control. Finally the number of male flowers per plant in the second experiment increased (the ratio of female to male flowers reduced), this is due to longer days and higher temperatures in spring than autumn. Keywords : Spermidine, methyl jasmonate, sex expression, cucumber, yield.