The experiments were conducted to evaluate effects of aspect ratio on wall and bed shear forces in rectangular channels. The objective was to determine contribution of wall shear force on total boundary shear force. A Preston tube connected to a differential pressure transducer and a data acquisition system was used to measure velocity heads at channel wall and bed. The calibration curves proposed by Patel were used to convert the pressure readings to shear stress. The results were adjusted by the total shear force calculated from the energy method. A nonlinear regression-based technique was carried out to analyze the results and develop equations to determine the percentage of wall and bed shear force on the wetted perimeter of rectangular channels. The proposed equations were compared and well correlated with previous studies. The new data has significant contribution on understanding and development of new relationships for boundary shear force.