This study measured automatic walking synchronization and how it associates with social impression. Previous studies discovered positive social consequence of motor synchrony with ecological paradigms (e.g. body movement synchrony between therapists and patients in clinical sessions, and the synchrony of side-by-side walkers). However, most studies of joint movement with high ecological validity face the same challenge, namely that conversations between participants might be the main or a partial contributor to the observed social benefits, as conversation is well documented to promote understanding and motor synchronization. We addressed this issue by using a novel paradigm to remove the conversation component and examined how synchrony per se interacted with social impression. Participants were paired to walk side by side in silence (i.e. without conversation) and their social impression toward each other was rated before/after the paired walk. Our results showed that walkers' first impression was positively associated with their step synchronization rate in the silent paired walk. Together with past findings, the bi-directional relation between body entrainment and social functions suggests that implicit nonverbal communication plays a significant role in providing a basis for interpersonal interaction.