Objectives: The objective of the present study was to compare the use of Hirudoid immediately after surgery with its use 6 hours post-operatively when probable thrombosis was formed but tissue damage was still reversible. Methods: After preparing 3 × 11 cm dorsal flap in all rats, one group of 8 rats received Hirudoid topically immediately after surgery during 9 post-operative days. Another group received it 6 h after surgery during 9 post-operative days. The control group received Vaseline immediately after surgery on a daily basis during 9 post-operative days. Results: The mean area of flap survival in the control group was 8.75 ± 3.32 cm2 (mean ± SD) and it was 12.38±1.93 cm2 and 14.36 ± 3.51 cm2 in Hirudoid immediately after surgery and 6 hours after surgery groups, respectively. Conclusions: It was found that Hirudoid can be effective in improving flap viability; although its effect was not statistically significant when used immediately after surgery (P > 0.05), it significantly increased flap survival when used 6 h after surgery (P = 0.003).