This study deals with the effect of uncertainty of activities' duration and the number of crews on the total project duration of repetitive projects. Based on the most optimistic (a), most pessimistic (b), most likely (m), and predicted durations (t) of the Program Evaluation Review Technique (PERT) and using the Line of Balance (LOB) method. Six potential scenarios for activities' duration uncertainty were provided in an attempt to model various situations of uncertainty that a repetitive project may face. The number of crews was calculated based on the deadline of the project and the rate of work in each scenario. A case study project of ten repetitive units with six activities each was studied for all the six scenarios. In each scenario, the LOB charts were created and the delivery durations of all units and the total project duration were calculated. The results of all the scenarios were compared, showing that the most optimistic scenario has the fastest delivery duration for the total project, while the deterioration and most likely scenarios have the slowest delivery durations for the total project. Based on the six proposed scenarios, the mean (µ) and the standard deviation (σ) were calculated. Hence, based on any reasonable statistical distribution, the probability of finishing the project at a certain time could be calculated.