Background: Newborn gender ratio is one of the most important factors in livestock industry. This ratio has affected on economic condition and controls sex-linked genetically transferred diseases. Offspring sex ratio is affected by such various factors. One of these factors is the Y to X bearing sperm ratio in normal semen that is called primary sex ratio, season and individual differences are important factors that can have an effect on primary sex ratio. OBJECTIVES: The objective of this study was to investigate two main effective factors, Individual difference and season, on proportion of sperms containing Y-chromosomes to sperms containing X-chromosomes in bull semen. METHODS: Semens were collected from 16 bulls during summer (Jul-Aug) and winter (Dec. to Jun). After sperm analysis, DNAs were extracted and Real-time PCR performed using three sets of primers to study SRY (Y-chromosome specific sequence), PLP (X-chromosome specific sequence) and PAR (Housekeeping) genes. RESULTS: The results showed that the sex ratio of sperms in normal ejaculation was not equal in different bulls.  Moreover, season can affect primary sex ratio. Y-bearing sperm increase in summer. CONCLUSIONS: We concluded that the primary sex ratio is affected by individual differences and season. We also suggested using Northern blotting on PCR products to confirm results.