A post chemiluminescence(PCL) phenomenon was observed when unsymmetrical dimethyl hydrazine(UDMH) solution was injected into a solution after the finish of CL reaction of alkaline luminol and potassium periodate. A new method for the determination of trace UDMH was established based on the studies of the CL reaction conditions including the concentration of potassium periodate and luminol,the length of tube,the velocity of solution flow,the luminal alkalinity(NaOH) and other experiments. The linear response range of this method was from 1.0×10^-6 to 1.0×10^-4 g·L^-1 with a detection limit of 3.3×10^-7 g·L^-1. The relative standard deviation for 4.0×10^-6 g·L^-1 UDMH solution was 2.7% for 11 tests. This method has been used to determine UDMH in water with satisfactory results. The PCL mechanism is also discussed briefly.