Preparation of benzaldehyde by Swern oxidation of benzyl alcohol was carried out in a continuous flow microreactor system. Dimethyl sulfoxide (Me2SO) was used as oxidizing agent and oxalyl chloride or p-toluenesulfonyl (p-TsC1) chloride was used as the activating agent. Benzyl alcohol was oxidized to benzaldehyde by the Me2SO activating agent mixture in the continuous flow microreactor system. The optimized reaction conditions of the Swern oxidation were as follows: oxalyl chloride was used as the activating agent; the mole ratio of Me2SO, oxalyl chloride, and benzyl alcohol was 4:2:1; the flow rate of Me2SO was 1.5 mL/min; the reaction temperature was 15 degrees C; length of delay loop was 1.5 m; a Caterpillar Split -Recombine Micro Mixer was used; and all of the experiments were completed at atmospheric pressure. The yield of benzaldehyde can reach 84.7% with selectivity of 98.5%. Due to the small reactor volume and short residence times, the Swern oxidation of benzyl alcohol in a continuous flow microreactor system can be operated at nearly room temperature (5-19 degrees C) instead of 70 degrees C in a batch reaction, with residence time of reactants in microreactors in milliseconds instead of several hours in a batch reaction.