Researchers intend recently to use LPG whose main composition is the liquefied n-butane for a CI engine. One of the capabilities is the dual fuel injection system, that is, the injection of LPG at the inlet port and that of gas oil in the cylinder. The flash boiling phenomena surely occurs in the case of the inlet port injection. Then, the experiments on the characteristics of the liquefied n-butane spray were carried out in the low pressure field under the atmospheric pressure with the room temperature. The experimental variables were the back pressure, the injection pressure and the injection quantity. The relative fuel concentration was obtained on the instantaneous 2-D image of spray by processing. The droplets distribution was measured on an enlarged photograph. And the clear information are obtained from these experimental results.