The market of light-emitting devices (LED) is growing dramatically over these years. To test the quality of LEDs, lots of LED probes are required. Therefore, it is important to develop an effective method to measure the angle (around 10 degrees) and the radius (15 μm ∼ 30 μm) of a produced LED probe. In this study, we propose a new subpixel edge detection for LED probes. The proposed method mainly consists of a coarse edge detection by Canny operators and a fine edge detection by a reconstructive method. In addition, an Otsu thresholding and a reflecting-point removal are included to reduce noise and increase accuracy. Compared to the previous methods, the proposed method can further reduce angle error up to 19.5% and the radius error up to 24.8%, respectively.