This paper considers a genetic algorithm for sequencing activities and allocating resources to reduce the over all completion time of workflow in the presence resource constraints. The algorithm provides an integrated solution for two sub-problems. The first is to decide the priority for the activities which require the same resource. The other problem is to select one among available resources for each activity by considering the incurred setup time and the performance factor of each resource. We evaluate the algorithm performance for three different kinds of workflows including parallel structures. Computational results show that the proposed algorithm is more effective than a previous work.