It is very costly if a software project development has to recover from an error that is due to a mistake made in the construction of the requirements model. Validation of requirements model is thus always an effective means for detecting defects in the requirements model. In this paper, we present an approach to modeling requirements by UML with OCL, and the design of a tool EOC (Executable OCL Checker), that supports well-formed static checking for the OCL specification as well as the dynamic validation. We illustrate the approach and the tool by an example of B2B system. The requirements model of this system is validated by the prototyping executions of the system functions on the system states against the OCL constraints. The validation checks whether the execution of a use case violates the system invariants, and whether the requirements model is feasible according to its business workflow. © 2009 IEEE.