This paper explores the possibility of improving the utility of the well-known WASP computer package for investment planning in power systems by introducing a Network Planning Model for representing and planning the power transmission network expansion in the WASP package as a module. The Network Planning Model formulation for power generation and transmission system planning is described in detail illustrated by an application for the Northern regional power network in India. The integration of the Network planning model with the existing WASP modules at various levels is suggested and the additional data input on the locational aspects of the generating plants and the topology of the transmission network is specified. The implementation of the Network planning module within the WASP package is expected to provide a capability for simultaneous optimization of the generation and transmission system expansion to the WASP model.