Many techniques have been developed to analyze the time impact and the costs caused by the disruptions of the project works. However, most were limited on the delay of the critical path and the overall project completion date. In recent years, although more attention has been drawn on the impact of the float loss on the overall project cost, not a single research result has been widely accepted and recognized. This study aims to fill this gap. This research firstly introduces a resource-integrated genetic algorithms (G.A.) model, which will be used to develop an optimal schedule including the timing and quantity for each type of resource required to complete each individual activity. Secondly, by using a case study this research intends to identify the impact of float loss on overall project cost through comparing the as-planned optimal schedule with an after-impacted schedule. Based on the research results this research has identified that the relationship between the float loss and project cost can be expressed as a step function and the time span of total float can be divided into and cost-related float and cost-unrelated float, termed as optimal float, in which the consumption of float will not cause the delay of the project completion date, nor the increase of project cost. The findings refute the traditional concept that the total float belongs to neither the owner nor the contractor, and suggest that the utilization float is free only within the optimal float and the float apportionment will never be reasonable unless the cost of float floss has been taken into consideration.