Abstract: The reproduction function of gradation is an important base in reproduction of color image.In this paper, the reproduction function and curves of gradation are researched, which is based on that print gradation is true to scenery. In the course of the research, the characteristic of light sensitive material and the whole process of image reproduction are considered. The whole research includes three aspects as the following. First, the printing curve of gradation true to scenery is deduced based on the characteristic curve of light sensitive material. And a function simulating the reproduction of gradation true to scenery is researched. Printing curves of gradation based on different originals' or prints' densities can be drawn by the function that has been founded. Then, the reproduction function of gradation in color separation is researched, which is on the basis of the simulating function of reproduction of gradation, the equation of gray balance and the formula of Yule-Nelson. That is to say, the function relationship between the densities (D_e) of four-color plates and the original density (D_O)can be founded by the function simulating the reproduction of gradation that has been founded and the equation of gray balance, and then, the function relationship between the dot percentages (F_e) of four-color plates and D_O can be founded from the relationship between D_e and D_O and the formula of Yule-Nelson. Finally, the reproduction curves of gradation in color separation is deduced in circle, based on the curves of gray balance, printing curves of gradation. In fact, the reproduction true to scenery not to original can be done, and the print will be more suitable to the habit of people's eyes.