We have already proposed a colorization method in RGB color space, where the colorization problem is formulated as the maximum a posteriori (MAP) estimation of a color image given a monochrome image. Markov random field (MRF) is used for modeling a color image which is utilized as a prior for the MAP estimation. In this paper, a colorization method in YCbCr space is presented, which is derived by the same formulation as in RGB color space. The presented method in YCbCr space is much simpler than that in RGB space and requires much less computation time: about one fourth of computation time in RGB space. As for quality of estimated color image, both methods in YCbCr and RGB space produce color images with comparable PSNR values.