The effect of venturi tube shape on reaction rate by hydrodynamic cavitation was investigated. The apparatus mainly composed of the venturi tube, a liquid pump, a tank, a pipe arrangement, and a heat exchanger. The upstream and downstream pressures of the venturi tube were measured. The sample was an aqueous solution containing potassium iodine and chloroform. The generation rate of triiodide ion increased with increasing upstream pressure.This is because liquid velocity at the venturi tube is high and the amount of cavitation bubbles increases. When the throat diameter of the venturi tube was 2.5 mm, the generation rate of triiodide ion reached a maximum value. As the throat diameter of the venturi tube became smaller, the generation rate of triiodide ion per pass of venturi tube increased and the pass number of venturi tube decreased. Both low cavitation number and large pass number are important for high reaction performance.