A new method for particle-size measurement of digital holography by Hough transform is proposed. The numerically reconstructed images of a particle-field hologram are approached in a way that in-focus images of particle in the volume are integrated into a plane and the edge of particle image is extracted, then by using Hough transform, the particle information of the size and the location is obtained. Computer simulations of the 2D and 3D particle fields are performed. The experiment is conducted for the field of diameter 90 μm standard particle, and the measuring absolute error is 6 μm. The research results show that the method is feasible.