This paper proposes novel stability analysis for variable step size incremental resistance INR maximum power point tracking MPPT. One important factor which judges system dynamics and steady state performances is the scaling factor (N), which used to update the controlling equation in the tracking algorithm to determine the new duty cycle. The main contribution of this analysis appears in developing the overall small signal model of the PV system, which consists of PV module, DC-DC boost converter, digital INR MPPT, and resistive load. The overall system model has been developed. Therefore by using linear control theory, the boundary value of the scaling factor has been determined. The theoretical analysis and the design principle of the proposed stability analysis have been validated using MATLAB simulation, and experimentally using digital signal processor DSP.