The resist filling behavior is crucial to the quality of the final imprinted pattern in micro/nanolithography. To obtain resist flow fields, a three-dimensional (3-D) defocusing digital particle image velocimetry (DDPIV) system was set up to realize the visualization of the resist filling process. Three-dimensional motion trajectories and velocity vectors of tracer particle sets were calculated to characterize this microscale flow phenomena. The results show that the particles’ maximum velocity is located in the region below the mold corner, and the region leans to the mold cavity. Meanwhile, the squeezed resist will first move along the mold moving direction, and then fills the mold cavity in an upward moving tendency when it passes through the mold corner. The feasibility of using the DDPIV system to investigate the resist filling behavior was confirmed. The results will help to understand the resist filling behavior and provide useful instruction for the selection of process parameters and mold structure optimization.