This paper addresses the problem of perceptual quality measurement on individual coded video frames with distortions caused by both lossy source coding and lossy transmission. We focus on low bitrate H.264 coded video with error concealment and propagation due to packet losses. Identifying Human Visual System (HVS) masking effect as one of the most important factors affecting the perceptual quality, we investigate its impacts on the two aforementioned artifacts in individual video frames respectively, and then evaluate their corresponding Just-Noticeable-Difference (JND) based perceptual distortions. Finally, we combine the two distortions together and propose a block-wise full-reference quality metric for a single distorted video frame. Our subjective evaluation results show that the proposed metric correlates fairly well with actual subjective quality ratings.