The optimum dose of UV irradiation to produce gynogenesis rainbow trout, Oncorhynchus mykiss with emphasis of Hertwing effect and photoreactivation (PR) was investigated. For this purpose, the sperm of rainbow trout were inadiated with UV at 2010±200 µw.cm-2 intensity in different alternatives of 0, 1, 3, 5, 8, 10, 15, 20, 25, 35 and 45 minutes and allowed to fertilize normal ova; the fertilization, eyed and hatching rate were calculated to assess the performance. Using the irradiated sperm decreased the fertilization, eyed as well as hatching rates and the so-called "Hertwig effect" was observed, with the time-dependent decrease in hatching rate at 0+to 10 min of irradiation, but a better hatching rate at greater time of irradiation was observed. The highest hatching rate was observed in 20 min of UV irradiation; after that the survival rate decreased rapidly. Interestingly, irradiation even up to 45 min could not eradicate sperm fertility and a hatching rate above zero was observed at this treatment. For PR studies the semen was irradiated with UV for 5, 30 and 120 seconds and untreated semen (Os) was used as control. Irradiated semen and/or fertilized eggs by treated semen were exposed to visible light (60 W) at a distance of 30 cm for 10 min; the eyed and hatching rates were measured. The results showed that UV irradiation in as low as 5s could cause deleterious effect on semen chromatin and decrease the eyed and hatching rates (p < /em><0.05). Illumination of semen andor eggs with visible light resulted in PR in rainbow trout when sperm was irradiated by UV at 120s. We could conclude that the best UV irradiation time for production of gynogenesis rainbow trout was 20 min and UV irradiation, fertilization and egg hardening must be done far from visible light to eliminate the PR mechanism.