Abstract: This study was carried out to determine the effects of egg weight and weight losses in incubation on hatching results of broiler parent stocks Cobb-500 and aged 35-36 weeks. Unhatched egss checked, embrionic mortality (early, middle, late and pipped period), hatchability of fertile egss and hatchability of total eggs and discarded chicks were determined with weighting eggs at the beginning and 18 day of incubation to record weight losses of eggs. According to results, embryonic mortality at early and middle period of development was not changed by egg weights group. However, this rate were 2.96 % in heaving eggs and 1.38 % in normal sized eggs in weight. The mortality rate were significantly different between egg groups at the period of late embrionic development (18-21 day). Egg weight did not affect the rate of pipped and discarded chicks (p>0.05). With respect to hatchability rate of eggs, there was significant difference between heavy ( 89.57%) and normal eggs ( 91.65%) (p<0.01). The egg weight losses were determined as 10.64% and 10.60% in normal and heavy eggs respectively (p>0.05).