The Lenselet-Based Plenoptic has recently drawn a lot of attention in the field of computational photography. The additional information inherent in light field allows a wide range of applications, but some preliminary processing of the raw image is necessary before further operations. In this paper, an effective method is presented for the rotation rectification of the raw image. The rotation is caused by imperfectly position of micro-lens array relative to the sensor plane in commercially available Lytro plenoptic cameras. The key to our method is locating the center of each microlens image, which is projected by a micro-lens. Because of vignetting, the pixel values at centers of the micro-lens image are higher than those at the peripheries. A mask is applied to probe the micro-lens image to locate the center area by finding the local maximum response. The error of the center coordinate estimate is corrected and the angle of rotation is computed via a subsequent line fitting. The algorithm is performed on two images captured by different Lytro cameras. The angles of rotation are -0.3600° and -0.0621° respectively and the rectified raw image is useful and reliable for further operations, such as extraction of the sub-aperture images. The experimental results demonstrate that our method is efficient and accurate.