Background: Sexual dysfunction is an important side-effect after radiotherapy (RT) for prostate cancer (PCa). The aim of this study was to compare sexual functions of PCa patients before and after intensity-modulated RT and to analyze their correlation with penile bulb (PB) doses and patient characteristics. Materials and Methods: Forty-two patients who underwent RT ± hormone therapy for PCa between 2010 and 2013 were analyzed. Sexual functions assessed by patient-reported questionnaire and physician reported scale before and 3 years after treatment. The effect of patients' age, prostate volume, testosterone levels, comorbidity, smoking status, tumor stage, RT technique, hormone therapy, and PB doses to sexual functions were investigated. Results: After 3 years of RT, 64.3% of all patients had a lower erectile score; and 75% of patients who were previously potent (n = 24) had become impotent after treatment. However sexual desire still remained in 75.8% of patients who had desire before treatment (n = 33). Statistical analysis showed that two parameters were correlated with postradiotherapy impotency outcome; PB mean radiation dose (P = 0.033) and testosterone levels (P = 0.032). Conclusions: RT, despite modern techniques, affects the sexual function of PCa patients in varying degrees. Reducing radiation doses to penile structures may play a role in preventing erectile dysfunction.