This paper presents an online ergonomic evaluation system for 3D product development with car interior design as an example, which consists of a 3D viewer, a digital human model, an ergonomic engine, and the web-based GUI's. The digital human is constructed with a number of templates based on anthropometry database that represent various levels of body size and shape for the end-user. The interactions between the human and a product model are captured by the viewer, and thus, simulate the user operation of the product. According to the Chaffin's biomechanical model, the ergonomic engine then computes the physical loads of body joints with the captured information. The result enables online evaluation of the product design from the ergonomic aspects. It also serves as a base of interactive product customization. This research is the first study that realizes the web-based ergonomic evaluation for 3D car interior design with no needs of high-end CAD systems or complex VR environment. In this manner, the human factor issues can be effectively taken into account at the early design phase and the costs of ergonomic evaluation will be significantly reduced.