This study presents the effects of adding bypass holes in a dry powder inhaler on aerosol particle characteristics. A commercially available inhaler device was modified with bypass holes, and the resulting drug powder spray characteristics were investigated in terms of how the direction, size, and position of the bypass holes in the inhaler passage affect the spray output to find the optimal design. All three factors influenced the aerosol behavior, and in particular, it is most significantly affected by the direction of bypass hole faces. Adding bypass holes to the inhaler passage was found to decrease the mean penetration length of the drug particles by 12%; however, at the same time, the spray angle is increased by 35%, which leads to a significant decrease in forward particle momentum.