The purpose of this work is to provide to the designer an iterative method of task temporal parameter adjustment in order to select the fitting characteristics for all the tasks obtained during the architectural design phase. We then present a method to analyse and to set the temporal parameters of tasks in a Rate Monotonic scheduling environment. So we have introduced a graphical representation for tasks based on a classical temporal model. This approach is particularly useful in situations in which the designer has to build a task set parameters iteratively, to add a task to a set of tasks with already fixed parameters or to modify the task parameters in order to improve time characteristics. This methodology permits the designer to enhance the temporal performance of the application in terms of task response time, processor idle time, number of processor context switches, etc..