Planning is one the most fundamental steps of project management. Every project consists of a set of activities. With the increase in number of activities or complexity of projects, project activities efficient management is important in the defined range of resources. Due to the limited organizational resources, activities planning is an issue for managers & researchers to be concerned about. A project should be accomplished in the specified range of time & budget. Therefore planning the project with realistic attitude is a necessity. Knowing the priority & the importance of the activities leads to efficient activities management & correct and accurate allocation of time, budget, cost and other resources. Thus, different elements such as cost, time, project staffs' skill & other elements can affect each activities weight factor. Determination of activity weight factor depends on the various aspects. To this end, current study has proposed an approach to determine activities weight applying Data Envelopment Analysis. The proposed approach includes three phases. In the first phase project activities have been extracted based on the work breakdown structure. Then, the second phase does recognize effective parameters on activities importance. In this phase, research's literature has been reviewed & these parameters have been extracted. Afterward, based on the academic and industry's expert's idea in the field of project management effective parameters on project activities importance have been selected. In order to assign weights to the activities in the third phase, initially the proper DEA model has been chosen. Model's inputs & outputs have been determined & activities weights have been specified on the basis of gained efficiency numbers. Eventually, the proposed model has been solved & demonstrated in a numerical example.