Abstract: One of the primary objectives of a phase I study is to evaluate toxicity at different dose levels and recommend the maximum tolerated dose (MTD). We give an extend up-and-down design method based on the isotonic regression estimation for identifying MTD. Through a lot of simulations, it shows that the design based on isotonic regression for identifying the maximum tolerated dose has improved the accuracy of recommending MTD. It apparently protects patients efficiently and prevents patients from excessive trials under the higher toxicity dose level and makes a great significance in phase I clinical trials design.