Continuous observation based on the operator's clinical experience is impractical in monitoring free flap and the success rate of flap can be increased with the adjunctive monitoring system such as laser Doppler flowmetry. The author tried to establish the objective critical point to analyze the data in the postoperative period in microsurgical reconstruction. In this study, laser Doppler flowmetry(Periflux 4001 Master , Sweden) was used continuously for flap monitoring from the recovery room; every hour for 24hours, every 2 hours in the next 24 hours and every 3 - 6hours thereafter until the 7th day. Among 43 cases performed, 4 patients suffered from postoperative vascular insufficiency and they were decided to follow the exploration or salvage procedure. The value may be recorded at most 3.0 even when the tissue sufferes without flow due to its highly sensitive system, and it is quite reliable for flap survival if the initial value is above 3.0 with increasing trend. If the flow value persists below 3.0 continuously or show a plateau, vascular compromise can be doubted. Initial plateau or decline curve was caused by flap swelling and could be detected in 2 postoperative periods such as early (2-24hour) and /or late (26 -116 hour) period. Its decline was maximally up to 31.5% and this decline was more commonly observed in perforator- based free flap than others. However, its appearance and duration was quite unexpected especially in small free flap due to skin tension after suture. In conclusion, the laser Doppler flowmetry is a valuable adjunctive system in flap monitoring after operation by observing the trend in perfusion value.