Currently, antimicrobial effectiveness parameter is predicted by measuring the inhibition zone diameter manually. In this paper, an antimicrobial effectiveness measurement system was introduced using circular Hough transformation method. The hardware of measurement system consists of a set of measuring workbench, two halogen light sources and a camera that connected to PC via USB. The software of system consists of image acquisition, camera calibration, pre-processing, Hough transforms and meter conversion. A checkerboard with known dimension and shape was used to calibrate the camera and to generate spatial calibration parameter. A contrast enhancement was performed by adding object background using strong color. The morphological method was used to detect the edge of the inhibition zone boundary. Then, the edge was used as an initial point to construct the circular shape using Hough transformation method. An antimicrobial effectiveness was measured based on the diameter alteration of inhibition zone which formed by an antimicrobial in agar subtract. The average measurement error of inhibition zone obtained from the antimicrobial object was 1.05% and 1.09% at camera-object distance 12.2 cm and 17.2 cm, respectively. Finally, the proposed antimicrobial effectiveness measurement system could be used as an instrument for antimicrobial effectiveness testing.