A new 3-dimensional (3-D) resist profile simulator has been developed. The energy distribution in the photoresist is calculated by Yeung's vector model in order to take into account the effects caused by oblique propagation of the light, which cannot be neglected because of high numerical aperture (NA) of the projection lens. A new algorithm for the development process is proposed to overcome the inconveniences of conventional models. A new differential equation which we call `Profile Equation' is derived and solved numerically to give the final resist profile. This model has been found to give practically the same results as those obtained by using string model in the 2-D case and can easily be applied to the 3-D case.