In the North of Persian Gulf region, sea urchins Echinometra mathaei are dominant in patches along the rocky coasts between the average low tide and a maximum depth of 5 m. In the current study, it was monitored monthly 217 individuals from Bandar Lengeh (26˚32ʹ28̋ N, 54˚52ʹ28̋ E; water depth: 0.5 to 1 m) during March to September, 2014. Their genders were obtained by dissection. The aim of this study was to determine the sexual ratio in their population and to see if they effect the operational sex ratio (OSR). The number of males and females were 71 and 126, respectively. Twenty samples by the total wet weights less than 12 g were non-sexual. The wet weight of the gonads was ~8.25% of the mean total wet body weights. The most distribution of gonad wet weights and total wet weights were generally less than 10 g and 80 g, respectively. The females were heavier than the males (~1.77x). The heavier females had more gonads (direct relation, r=0.45) while the lighter males had more gonads (inverse relation, r=-0.52). The males’ total dry weights were < 20 g and > 40 g but females’ were between 10 to 40 g. The total wet weight was > ~2x (females) and > ~7x (males) the total dry weight. The wet gonad weights is ~3x of the dry gonad weight of gonads and the two-thirds of total wet weight is the moisture of gonads. In females, Interquartile Range for gonad moisture content (GMC) was lower than the males or gonads of males had less moisture in compare with females. The operational sex ratio of E. mathaei was female-biased (OSR=0.36). The maximum and minimum of OSR was recorded in September (0.46± 0.11), and April (0.11± 0.11), respectively. It was not significantly different during the spawning period. The monthly sex ratios were significantly different during the spawning times. It was also observed an unstable condition for sex ratio and deviated from 1:1. The maximum of sex ratio was in April (1.07± 0.03) as the start of Spawning and September (0.86±0.02) as the recovery of gonad. Therefore, OSR cannot be affected by sex ratio at least at the start of Spawning time (April) and may be influenced by other parameters.