The effect of scattered radiation on the transmitted gamma ray signal should be carefully taken into account because it certainly affects the quality of reconstructed tomography images. Therefore this effect was evaluated with different Counting modes and gamma energies. For less dense objects such as wood and plastics, the build-up effect can be ignored and (CS)-C-137 can yield the better result than Co-60. On the contrary, for the relatively heavy material such as oil and metal the scattered radiation effect of (CS)-C-137 source should be considered because the build-up effect is more serious than Co-60. It was also found that the image quality gets better when the count data of photo-peak were used rather than the total counts from the full spectrum. The image improvement technique using precise weight matrix calculation method was suggested to remove the influence of the scattered radiation on the reconstructed image quality. From the experiment, it was concluded that the effect of scattered radiation on the image quality is varied depending on the material composition and radiation energy. Before the implementation in the field, normally the information of the process material is available so the optimal measurement condition can be predetermined on the basis of this Study. The additional data processing method is also expected to improve the reconstructed image quality further.