Abstract: One of the key problems in cathodic protection of tank bottoms is the judgment of protection effectiveness on whole tank bottoms, especially the protection effectiveness of bottom's center. For lack of simple and economical methods to measure the bottom's potential, the potential distribution modeling is an important approach to the study the bottom's protection. According to the theory of stable current electrical field, the mathematical modeling of potential distribution on bottoms had been already derived and confirmed by author's lab. It was found that the protection potential was a function of the ratio distance in. form of cubic parabolic-like curve. In present paper the measurement of protection potential and the judgment of protection effectiveness were proposed, namely, in one-point method, two-points method, multi-points method and circumference-potential method and so on. These methods could be used to evaluate the effect of anodic field and to predict the potential of bottom's center easily without needs of full gauge PVC measurement tube buried under tank bottom. It was shown by some examples' analysis that the accuracy of these methods could fit into the request of daily inspection.