Since, cost and time are known as two main factors in civil engineering projects, the aim of this study is to optimize the relationship between the above factors. In optimization process, a cost tolerance is considered to achieve a wide-range of time-cost trading off while trapezoid fuzzy numbers are used for time and cost over the project activities. Preemptive goal programming approach has been utilized to develop mathematical model and a Weigh in Motion system (WIM) in Isfahan-Nain road has been selected as case study. The proposed model has been utilized using a wide range of cost tolerance and results revealed that the proposed model is capable to identify the relationship between cost and time in construction projects. Decision makers who are dealing with these kinds of problems are recommended to apply the proposed model to achieve the best situation of project scheduling plan.