Product line is nowadays well known as a representative method for reuse. In the product line, important assets are components. Although enough concerns were given of the product line, it was not accomplished to structure and specify a product-line component with variability. This paper presents an approach to specify components in the product line. The approach describes the static and dynamic structure of a product-line component and explains the behavior and concurrency of the component. The component information is separately described in the black-box and white-box using the Feature-Oriented Reuse Method(FORM). This research also formalizes the data on a component specification in the form of BNF. The specification is described through careful consideration for many different characteristics of the product-line component, so this paper helps to easily develop the components in the product line and to well comprehend how to apply a method for the product line.