We propose in this paper the study of a particular spatially partially coherent source applied to digital in-line holography of dense particle flow. A source with a rectangular complex coherence factor is implemented. The effects of such a source on the intensity distribution of the diffraction pattern are described. In particular, we show that this type of source allows us to eliminate the diffraction pattern along one axis while all the information about the dimension of the particle is kept along the other perpendicular axis. So particle images can be well reconstructed along one direction and the speckle can be largely limited.