Abstract: The common method for detecting small tubes is the focused beam immersion method. This method is highly sensitive and reliable, but not fit for in-situ inspection, due to low speed and high cost. To solve these problems, the method based on circumferential guided waves technique was proposed. In actual testing process, a series of special echoes, which were valuable for guaranteeing the reliability of the tests. It verified that these echoes were reflected waves by the lateral surface of couplant. The velocity dispersion curves for the guided waves in tube walls were calculated numerically and then the modes of the guided waves were determined, and the displacement profiles across the wall thickness were obtained similarly. The experiment and caculation results showed that large percent energy of the guided wave was concentrated near the outer surface of the tube, and it was useful for in-situ inspection of small tubes as a compliment to the focused beam immersion method.