Abstract: Onedimensional heat conduction equation of thermal wave during modulated heating was described. A method, correlation algorithm, was promoted to detect phase information in lockin thermography nondestructive testing in order to reduce sample images. Using the principle of the reference signal being uncorrelated to the measured signal, the latter could be extracted easily. Compared with averaging fourpoint algorithm, correlation algorithm was shown to have better precision and need less information. The experiment also confirmed that the phase was less affected by surface nonuniformity. So the phase thermography technique was expected to have wide applications in NDT field.