[Purpose] The purpose of this study was to analyze the architectural properties of muscles on ultrasonographic images of chronic stroke patients taken during different muscle activities. [Methods] Thirty chronic stroke patients were equally divided into three groups according to their Modified Ashworth Scale grade (1 to 3). Ultrasonographic equipment was used to measure structural properties of their skeletal muscles (muscle thickness, muscle pennation angle, and length of muscle fascicles). [Results] Muscle thickness, pennation angle, and fascicle length significantly decreased both at rest and during MVIC (Maximum Voluntary Isometric Contraction) as muscle spasticity increased. Each group’s muscle pennation angle markedly increased during MVIC compared to at rest. Each group’s muscle thickness and fascicle length decreased during MVIC compared to at rest. [Conclusion] Changes in structural properties of the skeletal muscles influenced architectural properties of the muscles on ultrasonographic images. Our results indicate that understanding the structural properties of the skeletal muscles of chronic stroke patients needs to take into consideration the ultrasonographic architectural properties of the muscles.