Multi-cameras calibration using spheres is more convenient than using planar target because it has an obvious advantage in imaging in different angles. The internal and external parameters of multi-cameras can be obtained through once calibrat ion. In this paper, a novel mult i-cameras calibration method is proposed based on multiple spheres. A calibration target with fixed multiple balls is applied in this method and the geometric propert ies of the sphere projection model will be analyzed. During the experiment, the spherical target is placed in the public field of mult i-cameras system. Then the corresponding data can be stored when the cameras are triggered by signal generator. The contours of the balls are detected by Hough transform and the center coordinates are determined with sub-pixel accuracy. Then the center coordinates are used as input information for calibrat ion and the internal as well as external parameters can be calculated by Zhang's theory. When mult iple cameras are calibrated simultaneously from different angles using multiple spheres, the center coordinates of each sphere can be determined accurately even the target images taken out of focus. So this method can improve the calibration precision. Meanwhile, Zhang's plane template method is added to the contrast calibrat ion experiment. And the error sources of the experiment are analyzed. The results indicate that the method proposed in this paper is suitable for mult i-cameras calibrat ion.