The function of gelatin is to stabilize and protect the catalytic activity of the enzymes in dried form, and to control the diffusion rate of the sample solution to match the overall color-forming process. Fig. 2 displayed the effects of gelatin concentration, optimized concentration of gelatin is 0.625%. According to our observation, with the lower concentration of gelatin the red color would expand fast with the water but with the higher concentration the water would not pass through and stopped at the bottom of the detection zone. They are both unfavourable for the color-forming.