In this paper, we propose an efficient solution to 6-d.o.f. localization using an unscented Kalman filter (UKF) for planetary rovers. The solution is a technique using a modified UKF for accurate planetary rover localization. The technique is able to cope with problems that occur frequently on other planetary surfaces: wheel slip, pulsive noise of visual odometry (VO) and gyro drift. To solve these problems, the technique estimates a slippage ratio with the UKF, the accuracy of the VO with the number of inliers of feature points and sensor usefulness with a gyrodometry model. Experimental results of rover runs over rough terrain are presented and discussed; the effectiveness of proposed method and its components is shown.