The Tin coating thickness has significant influence on the quality of the mobile phone case product. It is very difficult to obtain the Tin coating thickness using conventional testing method. This paper proposes a new technique for efficient and accurate measuring the Tin thickness plated on the surface of mobile phone case products through analyzing the section image of the product derived from SEM. A computer image analysis has been carried out to obtain the thickness of the Tin coating. In the paper, we describe the detailed sample preparing processes which involve the use of rubber container, clamp, PS resin, firming agent, polishing machine, 3000 mesh sanding paper, silk, SEM and other tooling, scanning the sample section with SEM to obtain the image in computer and analyzing the image in the computer. This new method is very efficient and quite accurate, thus can be used in the production line to monitor the Tin coating thickness to help control the product quality.