This paper describes the technical details of design and implementation of embedded system-based CAPTCHA (verification code) in C, including a brief analysis of techniques for implementing CAPTCHA, and a simple and practical CAPTCHA system designed by the author of this paper, which can be used not only in embedded systems but also ordinary Web based MIS or control systems, without the support of any dedicated graphics library. An example is provided to show the practical usage and effect of this CAPTCHA system on an embedded application demo system.