Stroke is the leading cause of long-term disability, and many stroke patients have hemiparesis. Hemiparesis induces ankle-control disturbances and equinovarus deformity, leading to difficulty in walking and an increased risk for falling. Plastic ankle-foot orthosis (PAFO) is frequently prescribed to correct ankle joint alignment and increase walking speed and stride length during ambulation. While several studies have shown that PAFO improves gait parameters, such as stride length and walking speed, in hemiplegic patients, the effect of PAFO on gait stability remains unclear. We quantitatively assessed the effect of PAFO on gait stability in 16 hemiplegic stroke patients (mean age 55.9 ± 11.8 years; 5 female and 11 male subjects; and 11 hemorrhagic and 5 ischemic stroke) using an ink footprint record. Wearing PAFO significantly improved the stride length, step length on the unaffected and affected sides, step width, walking speed, step frequency and functional ambulation ability. The coefficient of variation (CV), as an index of stability of movement from trial to trial, provides a measure that defines motor skills for a given task. Unaffected-side step-length CV and step-width CV were significantly decreased, when using PAFO. Furthermore, the correlation was found only between unaffected-side step length and its CV. The decrease in CV indicates that PAFO improved gait stability. We concluded that in addition to providing a faster gait, PAFO improves gait stability during walking. Gait stability and gait efficiency need to be considered separately in evaluating the effects of ankle-foot orthosis on gait performance in hemiplegic patients.