Measuring a perceptual quality of an image is one of the important tasks in various applications such as image coding, processing, enhancement, and monitoring system. Although active researches have been made for objective quality assessment of 2D images for some decades, still very few efforts have been concentrated on 3D image quality assessment. In this paper, we propose a new quality metric for stereoscopic images based on the binocular perception model considering asymmetric property of a stereoscopic image pair. Experiments for publicly available databases show that the proposed metric provides consistent correlations with subjective quality scores. The results also show that the proposed metric outperforms state-of-the-arts metrics.