This paper deals with design and implementation issues of a DSP-based single-phase shunt active power filter (SPSAPF) under distorted voltage source. A modified control strategy based on instantaneous reactive power theory (p-q theory) is proposed in this paper. Theoretical formulations for both original and modified p-q theories are presented. Simulations of the system in MATLAB\Simulink environment are conducted to validate the effectiveness and capabilities of the proposed control strategy. These simulations have been organized in 4 different cases to demonstrate some important features of the proposed method as compared with the results obtained using the original p-q theory. Moreover, experimental results based on a 1 kVA prototype single-phase pure shunt active power filter are presented to evaluate the performance of the proposed control strategy.