Abstract: Based on the fact that in an acidic medium of HCl, intense chemiluminescence (CL) was induced by the oxidative reaction between Ce(Ⅳ) and the hydrolyzate of dimethoate produced in an alkaline solution of NaOH, and on the application of sequential injection (SI) technique, a method of SI-CL for determination of traces of dimethoate in environmental water samples was proposed. The tri-zone injection mode was used and the related reagents and water sample were combined in 3 portions as follws: (a) 0.3 mol·L-1 NaOH solution and 8 g·L-1 HPC solution, (b) water sample and (c) 0.035 mol·L-1 Ce(Ⅳ) solution and 1.2 mol·L-1 HCl solution, with their injection volumes of (a) 200 μL, (b) 200 μL and (c) 80 μL. Values of intensity of CL was found to keep linear relationship with mass concentration of dimethoate in 2 ranges of 1 to 100 μg·L-1 and 100 to 800 μg·L-1, with detection limit (3σ) of 0.17 μg·L-1. Values of recovery found were in the range of 85.7% to 119%.