Mobile Web services enable the IT industry and the mobile industry to create products and services that meet customer needs within the existing Web services framework. Transaction support is crucial in mobile Web services. The existing mobile transaction models are application-specific and not suitable to investigate the generic theoretical model. We present a new formal method called PMTM (P system-based Mobile Transaction Model) to formalize the behavior of mobile transactions. There are two kinds of transition rules in PMTM. The first group is object rules, which describe the transitions in membranes. The second group is membrane rules, which define the structural modification of membranes. Finally, we use mobile dining philosophers problem to illustrate the usage of PMTM.