For a better understanding of the deposition mechanism of thin films in SiCl4 source gas, we have measured the spatial distributions of SiCln (n=0-2) radicals in SiCl4 radio frequency glow discharge plasma utilizing a mass spectrometer equipped with a movable gas sampling apparatus. The experimental results demonstrate that the relative densities of SiCln (n=0-2) radicals have peak values at the position of 10 mm above the powered electrode along the axial direction; the relative densities of the Si and SiCln (n=1, 2) radicals have peak values at the positions of 27mm and 7 mm away from the axis along the radial direction, respectively. Generally speaking, in the whole SiCl4 plasma bulk region, the relative density of Si is one order of magnitude higher than that of SiCl, and the relative density of SiCl is several times higher than that of SiCl2. This reveals that Si and SiCl may be the primary growth precursors in forming thin films.