Abstract: Digital image correlation (DIC) method and its reliability and accuracy are commonly accepted in the measurements of affine deformations since it obtains comparable results with strain gauges. However, in engineering measurements, there are always substantial local deformations with high strain gradients such as the Portevin-Le Chatelier (PLC) bands, deformations near gaps and crack tips. In these situations, because the results within the contact areas are smoothed, strain gauges are restricted. Here DIC method can be employed to solve this problem. Problems that the calculation parameters (the order of shape functions, template size, etc.) seriously impact on the results and often make it difficult to get reliable results. In this paper, the deep mechanism that how the shape functions and the size of the templates impact on the accuracy of DIC results is discovered via analyzing PLC bands with different gradients in tensile tests and simulated bands. Second-order shape functions are more suitable than first-order shape functions to describe local deformations according to the simulation results. A gist is proposed that the results of second-order shape functions are reliable and accurate when the relative error between first- and second-order shape functions is less than 10%.