Objective: Over-diagnosis and under treatment of varicocele may be responsible for poor outcome of varicocelectomy. In this study, we used Color Doppler ultrasound for accurate diagnosis and grading of varicocele and to predict the outcome of microsurgical subinguinal varicocelectomy. Materials and Methods: A total of 104 patients undergoing microsurgical subinguinal varicocelectomy for treatment of infertility were included in this study. Patients were evaluated with routine history, physical examination, semen analysis, hormonal assessment and scrotal ultrasound and Doppler. After varicocelectomy, improvement index, in sperm concentration, was calculated by dividing the difference between the postoperative and preoperative sperm concentration by the preoperative sperm concentration. Improvement Index greater than 0.5 is considered a good outcome. Statistical analysis was done to study the correlation between microsurgical varicocelectomy outcome and testicular vein diameter at the inferior pole of the testis and the degree of reflux measured by color Doppler ultrasound. Results: Improvement index in sperm concentration, motility and morphology more than 0.5 was achieved in 58.8%, 27.3% and 17.6% of cases respectively. We found that patients with testicular vein diameter, at the inferior pole of the testis, more than 2.5 have significantly higher improvement index in sperm concentration, motility and morphology than in patients with testicular vein diameter less than 2.5mm (p=0.006, 0.016, 0.041 respectively). We also found that patients with clear reflux detected by color Doppler ultrasound at the inferior pole of the testis have a significantly higher improvement index in sperm concentration, motility and morphology than patients with reflux detected only in the supratesticular venous channels (p=0.013, 0.015 and 0.045 respectively). Conclusion: Color Doppler ultrasound is a useful tool for accurate diagnosis and grading of varicocele and to predict the outcome of varicocelectomy. We recommend doing varicocelectomy in cases of testicular vein diameter more than 2.5mm and in cases of reflux detected at veins at the lower pole of the testis.