The importance of supplier involvement in New Product Development (NPD) has been emphasized. Suppliers can help makers adopt new technologies and improve the NPD performance in cost, quality, delivery,etc. On the other hand, makers may bene?t from an independent relationship with suppliers. These two kinds of view suggest different operational processes for the supplier involvement in NPD. A path model among the supplier partnerships, NPD performance, internal capabilities in R& D, manufacturing, and market was built and examined empirically. The customer partnership was examined as a comparison. A survey was conducted with a questionnaire in 156 manufacturing enterprises. The methods of factor analysis and Structure Equation Modeling were adopted for data analysis. The results showed that, although an intermediary role of internal capabilities including R& D, manufacturing, and marketing, is necessary for the impaction of the supplier partnerships on NPD, it is not necessary for the customer relationships. The results were discussed from the view of endogenous growth view, which suggests that without the support of internal capabilities, it would be difficult to improve NPD performance with the supplier partnerships. Some suggestions were provided on how to improve the endogenous growth capability with the help of the supplier involvement.