Testicular development of bulls, during the post-weaning period, is associated to the age and breed of the animals, environmental conditions, and nutritional regime. A positive correlation between testicular development and fertility has been documented through numerous studies (Bailey et al. 1996). Scrotal and testicular measurements, which are simple and inexpensive to obtain, have been used to predict sperm production and semen quality. However, because SC is an indirect measurement of testicular mass which discounts variation in scrotal wall thickness and individual testis shape, the length and width measurements could provide a useful tool for predicting bull fertility. Therefore we conducted a study to evaluate testicular growth during puberty and to determine 1) appropriate models for analysis of phenotypic relationships of testicular dimension and growth traits to age during puberty; and 2) correlations of testis width and length to plasma testosterone and body weight in pubertal Holstein bulls under environmental conditions of Iran.