The Image quality of a mobile display in outdoor environment is evaluated in quantitative methodology. Contrast and colorfulness were chosen as the main quality factors among several quality attributes such as sharpness, lightness, noise and naturalness, and expressed in Just Noticeable Difference unit to correlate the image quality metric with human visual system. The overall image quality metric is determined by two quality metrics using contrast and colorfulness, respectively. These two quality attributes are obtained from measured XYZ values of the test image on mobile display in outdoor environment.