BACKGROUND: The potential risk of vascular complications associated with heparin, the dose of heparin therapy has not been exactly examined in patients undergoing transradial angiography. Thus, this study was aimed to compare referral arterial thrombosis, hematoma and hemorrhagic complications with 2500 and 5000 IU arterial heparin and the association of these complications with predictors in patients undergoing diagnostic angiography. METHODS: This prospective, randomized, double-blind controlled trial was carried out on 441 patients aged ≥ 18-year-old in Isfahan, Iran. They were referred for diagnostic coronary angiography with radial access. First participants were randomized into to inject either 2500 IU (group A) or 5000 IU (group B) of heparin. Study’s primary endpoints were thrombosis, hematoma, and hemorrhage. RESULTS: The frequency of thrombosis was 25.5% in group A vs. 2.3% in group B (P < 0.001), while the frequency of hematoma had no significant differences in group A and B. None of patients in both groups had hemorrhage. Using 5000 IU of heparin protected the occurrence of thrombosis by 95% [odds ratio (OR): 0.05, 95% confidence interval (CI): 0.02-012] after adjustment for confounders. CONCLUSION: The low dose (2500 IU) versus standard dose (5000 IU) of heparin use increased the risk of thrombosis following trans-radial diagnostic coronary angiography, with no effect on hematoma and bleeding.