The objectives of this study were to assess the effectiveness of clonidine in treatment of heroin withdrawal syndrome and to describe the side effects of clonidine. Methods: This study involved 171 in-patient opiate addicts who had withdrawal syndrome from opiate dependence treated at National Institute of Mental Health. This is a cross-sectional study comparing patients' symptoms before and after 7-10 days of treatments. Results: The results showed that clonidine significantly reduces withdrawal symptoms after 3 - 6 day: In day 1, withdrawal symptoms such as yawn (77.7 percent), insomnia (62.8 percent), cravings (62.8), muscular pain (49.) were slightly reduced. In day 6 of treatment, withdrawal symptoms such as yawn (1.7 percent), insomnia (14.8 percent), cravings (0.8 percent), and muscular pain (21.6 percent) were significantly reduced. Withdrawal symptoms continued to decrease from 7 to day 10, where the withdrawal symptomatic decreasing trends stopped and the patients were completely recovered. There were few side effects in the effective low dosage. High dosages of clonidine (0.9 - 1.2 mg), 18 percent of the patients experienced some side effect but these side effects rapidly disappeared after a week of treatment. Conclusions: clonidine is an effective drug in treating opiate withdrawal syndrome.