Abstract: A new calibration method for gain and phase errors in uniform rectangle arrays for underwater 3-D sonar imaging systems is presented. By setting a calibrator source at an unknown position in the far field, an efficient and fast three-step-iteration (TSI) algorithm is first proposed to provide a robust direction estimator in the presence of gain and phase errors. The gain and phase errors are then estimated based on the obtained direction estimation and the sampled spatial covariance matrix. With the obtained estimation of array errors, the 3-D sonar imaging systems can perform well even when the gain and phase errors exist. The calibration performance is compared with the Toeplitz-block-based approach through simulation. Experiment results show that the proposed method can achieve better performance when the SNR is small, and remains to be effective under large phase error conditions where the Toeplitz-block-based approach cannot work.