In this paper, a DCT based JND model for monochrome pictures is proposed. This model incorporates the spatial contrast sensitivity function (CSF), the luminance adaptation effect and the contrast masking effect based on block classification. Gamma correction is also considered to compensate the original luminance adaptation effect which gives more accurate results. Moreover, a psychophysical. experiment was designed to parameterize our model. Experimental results show that the proposed model is consistent with the human visual system. Compared with the other JND profiles, the proposed model can tolerate more distortion and has much better perceptual quality. The proposed JND model can be easily applied in many related areas, such as compression, water-making, error protection, perceptual distortion metric, and so on.