Scheduling distributed applications effectively and efficiently on Grid environments is difficult because of the dynamic and heterogeneous characteristics of the Internet. In this paper, we propose a loosely coupled application model for building distributed applications on Grids. We assume that a Grid application is composed of a group of independent modules. Each module performs either a remote service request or local processing. Different modules in such an application exchange information by explicitly described data that can be understood by both the application and the Grid environment. Each module is triggered by its input data, and finally it produces some output data. All information exchanges are completed transparently as they are carried out by the Grid management system. We call a module in such an application a loosely coupled module (LCM). A loosely coupled application can be defined by the combination of dependent or independent LCMs. By the loosely coupled application model, Grid applications can be built by employing discrete and heterogeneous resources on the Internet. The loosely coupled relationships among different LCMs can guarantee the robustness of the application. Parameters are defined in the application model so that application schedulers in the Grid environment can efficiently implement application scheduling by designing appropriate scheduling algorithms based on these parameters.