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Ball-pressure test is a primary approach to test the thermotolerance of non-metallic materials and solid insulating materials in electrical and electronic products. Accurate measurement of the diameter of the indentation is a key step in the ball indentation test, especially for the critical size of the indentation. The accuracy of the measurement directly determines the correctness of the test results. It is difficult to accurately identify the contact zone between the pressure ball and the indentation. Consequently, how to measure the size of indentation objectively, accurately, and quickly has become a major issue that has plagued quality personnel for a long time. The indentation image was captured under the perpendicular incident light. Since the contact zone and non-contact zone between the pressure ball and the indentation have different reflection characteristics to the perpendicular incident light, the contact zone can be accurately segmented from the non-contact zone by image processing algorithm. After the image is pre-processed, the grayscale center of gravity method is utilized to determine the gray center position of the indentation. The demarcation point of the contact area between the ball and the indentation is identified. Then, the pixel distance of the indentation is calculated through the Hough transformation. Finally, the actual size of the diameter is obtained via the calibration method. Repeatability test and accuracy comparison test results show that the repeatability standard deviation <0.0016mm, repeatability limit <0.003mm, absolute deviation of the measurements and the calibration results <0.01mm, which meets the required precision of indentation measurement.