[Purpose] The purpose of this study was to investigate the effects of treadmill training with a horizontal impeding force applied to the center of upper body mass on the gait and balance of post-stroke patients. [Subjects and Methods] Twenty-four subjects with hemiplegia less than 3 months after stroke onset were randomly assigned to 2 groups: an applied horizontal impeding force on treadmill training (experimental) group (n = 12), and a control group (n = 12). Both groups walked on a treadmill at a comfortable or moderate speed for 20 minutes per day, 3 sessions per week for 8 weeks after a pre-test. The experimental group also had a horizontal impeding force applied to the center of their upper body mass. [Results] All groups demonstrated significant improvement after 8 weeks compared to baseline measurements. In intra-group comparisons, the subjects’ gait ability (CGS, MGS, cadence, and step length) and balance ability (TUG, BBS, and FRT) significantly improved. In inter-group comparisons, the experimental group’s improvement was significantly better in CGS MGS, cadence, step length, TUG, and BBS, but not in FRT. [Conclusion] Treadmill training was identified as an effective training method that improved gait and balance ability. A horizontal impeding force applied during treadmill training was more effective than treadmill walking training alone at improving the gait and dynamic balance of patients with stroke.