A straining process of potatoes by a professional chef was examined by motion analysis. The force applied by him in the process was expressed as its vertical component, horizontal component and their resultant force. Their maximum forces measured were 11.35kgf, 2.92kgf and 11.83kgf, respectively, indicating that the vertical component was predominant over the horizontal component in the resultant force through the whole process of straining. This experiment showed that the motion analysis was well suited to explain a load on food materials dynamically and that the analysis would be useful for investigation of cooking-processes.