Myoglobin, which exists widely in skeletal muscle of vertebrates, plays crucial roles in life processes. Its proper folding depends on the binding of heme. In this paper, we report an atomistic and statistical analysis of force-induced unfolding of myoglobin with and without heme binding. It shows that heme is responsible for not only its biological functions, but also its unfolding dynamics. The unfolding pathway of myoglobin without heme binding involve an intermediate configuration. More importantly, it shows that intermediate state in force-induced unfolding process is different from that in chemical denaturant, which leads to the discovery of new intermediate configurations. Our results are in good agreement with related experimental observations and provide significant insight into general mechanisms of myoglobin unfolding.