The present work represents the effect of a coherent reference beam on the contrast of partially developed speckle pattern. An analytical formula for the average speckle contrast in the presence of a coherent reference beam is derived. The theoretical study was showed presence of a reference beam arises mutual interference with the scattered waves contributing in the formation of the speckle pattern. The produced interference pattern modulates the speckle pattern which decreases its contrast. The speckle contrast can be controlled by the ratio between the intensities of the reference and the scattered beams as well as the optical path difference between the reference and the object beam. Experimental work is carried out showing the speckle pattern is modulated by interference pattern due to the presence of a reference beam. It leads to decrease the speckle contrast with increasing the intensity of the reference beam with respect to the intensity of the diffused beam from the rough object. The experimental results are in good agreement with the presented theoretical study.