Business processes are often constrained by resource availability. This is particularly true in an emergency response system where large quantity of resources, including emergency responders, ambulances, fire trucks, medications, food, clothing, etc., are required. In our previous work, we introduced WIFA model to meet the needs of incident command systems workflow modeling and analysis. In this paper, we extend WIFA to take resources into account when modeling and enacting workflows. A formal resourceconstrained workflow model is defined and the state transitions rules are established to support workflow analysis. An example is given to illustrate the use of the model.