A calibration method for a camera which monitors a target board is disclosed, comprising steps: using the camera to capture a target image corresponding to the target board; using rotation and translation to align the target image to a reference standard and acquire the corresponding relationship of the target image and the target board; defining a region of interest above the target image; using at least one positioning marker in at least two positions of the target board, whose actual space coordinates are known, and using the camera to acquire at least two image coordinates of the positioning markers in the region of interest; and using the known actual space coordinates and the acquired image coordinates to work out the exact position of the camera. Thereby, the positions of the cameras can be used to correctly determine the positions of darts.