An intra-prediction mode optimization method and image compression method/device are disclosed. To properly determine an intra-prediction mode for a target block of an image, a control block is selected from reference blocks of the target block, and an intra-prediction mode adopted in the control block is regarded as a control mode. Assuming that the target block is encoded according to the control mode as well, the prediction efficiency of the target and control blocks is assessed. When the intra-prediction efficiency, under the control mode, of the target block is better than that of the control block, the control mode is determined to be the intra-prediction mode of the target block and an intra-prediction pattern of the target block can be obtained. Otherwise, another one of the reference blocks is selected to be the control block and the control mode is refreshed, to repeat the intra-prediction efficiency assessment.