A method for detecting two dimensional sketch data from source model data for three dimensional reverse modeling. The method includes the steps of detecting optional model data, establishing X-axis, Y-axis and Z-axis of the model data depending upon a reference coordinate system information inputted from a user, and setting a work plane for detecting two dimensional section data of the model data; projecting, on the work plane, two dimensional section data to be detected from the model data or polylines detected by designating a detection position; detecting two dimensional projected section data of the model data projected on the work plane, and dividing the two dimensional projected section data into feature segments depending upon a curvature distribution; and establishing a constraint and numerical information in accordance with connection of the divided feature segments of the two dimensional projected section data, and creating two dimensional sketch data.