The gonad developments of 217 sea urchins (Echinometra mathaei) were monitored monthly from March to September 2014 in the intertidal zones of Bandar Lengeh in the Persian Gulf. The numbers of males, females, unsexual samples were counted for 71, 126 and 20, respectively. Females were totally 1.77 times heavier than males. Wet weights of gonads were nearly 8.25% of the mean total wet body weights. Only females were observed by gonad wet weights more than 12 g. The gonad wet weights of females (3.74±3.26) was heavier than males (3.46±2.63). March to August was the spawning months, as a U-shaped trend by August (mid-summer) as its concave curve and it means that the most stores of gonads is in spring. The operational sex ratio (OSR) of E. mathaei was female-biased and was not significantly different during the study. The monthly adult sex ratio (ASR) was significantly different. It means that the sex ratio has no effects on OSR. Moreover, the gonadosomatic indices GSI 1, GSI 2, and GSI 3 were 8.2±0.6, 4.1±0.3 and 9.4±0.9, respectively. GSIs did not showed differ significantly in females; while in males, they were different. It was also obvious difference for males in spring and summer. All three indices were higher in the spring than summer (for example for GSI 2: 1.6 (August) ≤ 7.3 (March)).