The barbell squat is a commonly used resistance training exercise for the lower limbs that uses and strengthens the muscles surrounding multiple joints of the lower limbs, and as such is considered one of the most basic resistance training exercises. For this reason, previous studies have examined the relationships between quadriceps activity and stance width, knee flexion angle, or tibial rotation angle during the squat exercise. However, there is a lack of studies examining the effects of different stance widths and the effect on power productions during squat training. The purpose of this study was to examine whether using different width stances 　 influenced maximal power production while performing the barbell squat exercise. Eight trained male (means ± SD, age 21.1 ± 2.1 year, height 171.4 ± 6.8 cm, body mass 80.0 ± 13.4 kg) volunteered as subjects for this study. Squat power was assessed, utilizing four different stance widths while exercising with 60% 1 RM loads. Power was measured using an external dynamometer attached to the barbell, (Myotest, Inc., Switzer-land). Since the barbell COM and the body COM travel similar distances, system mass (body mass + barbell mass) was used for all power calculations. The squat stances used were 1) feet 50% bi-acromialthe, 2) feet 100% bi-acromial width, 3) feet 150% bi-acromial width, and 4) feet 200% bi-acromial width. For each of the four different stance widths, the subjects performed three reps at maximal concentric velocity. The concentric pahse started when the posterior thigh was parallel to the ground. The Friedman test was used to determine significant differences in power between the four stances. The level of significant for all statistical analyses was p < 0.05. Resulting power values were expressed normalized to power using the 100% bi-acromial width stance, and expressed as a percentage. Maximal squat power occurred using the 150% bi-acromial width stance (128.4 ± 32.5%), and was significantly higher than the squat power using 50%(97.5 ± 14.3%), 100% and 200%(101.2 ± 16.2%) bi-acromial width stances. These results indicate that altering the stance widths during the barbell squat exercise result in different levels of power production different. The effect of barbell squat stance width needs to be considered when coaches and trainers measure their athletes' 1RM or assess power production during that exercise.