Surface dynamics is among the most important problems of chemical physics. The definition of the structure of a surface is difficult and, unlike hulk properties, external fields play a significant role on surface. These external effects may result in structural ordering or disordering of the surface molecules. The phase transitions from ordered to disordered phases are controlled by order parameters. There are several definitions of order parameters; however, they mostly function if the surface and the bulk are studied together in three dimensions. In case of the systems which are studied in two dimensions, the traditional formulations of order parameters cannot be used. In this work, a 2D order parameter is developed which is in fact a projection onto two dimensions. The results of the proposed order parameter and the orthodox 3D one are compared for several models of membrane surfaces.