This paper proposes a hypothesis concerning the mechanism causing asymmetrical sprays from sac-less nozzles used for direct-injection diesel engines. The mechanism was deduced by using 3-dimensional numerical simulation techniques. It has been thought by many researchers that the eccentricity of the needle under a low lift condition causes the flow inside the nozzle to vary, resulting in an asymmetrical spray. However, the reason why such eccentricity occurs has yet to be clarified. In this study, the cause of needle eccentricity was investigated by using 3-dimensional fluid analysis techniques to calculate the force acting on the needle and a probable mechanism of asymmetrical sprays is proposed on the basis of the results.