Program verification is a promising approach to improving program quality. To formally verify aspect- oriented programs, we have to find a way to formally specify programs written in aspect-oriented languages. Pipa is a BISL tailored to AspectJ for specifying AspectJ programs. However, Pipa has not provided specification method for pointcuts in AspectJ programs. Based on the exist work of Pipa, and related issues, this paper proposes an approach to specifying pointcuts using purity conception in JML. This paper also provides several examples to illustrate our pointcut specification approach.