Modern component-based systems consist of active components that execute in parallel, which brings great difficulties in verifying correctness. By extending component calculus, a theory concerning refinement of active components is proposed. For interfaces, contracts are introduced which give functional specifications for both public methods and active action in terms of guarded designs. Then, a contract's dynamic behavior is defined by a pair of divergences/failures sets. The refinement relation between contracts is defined as the set inclusion of their divergences/failures sets. The theories applying simulation techniques to assure the refinement relation are proved. By defining the semantics of a component as a mapping from the contract of its required interface to the contract of its provide interface, component refinement can be proved in terms of contract refinement. When the component- based systems are being constructed in a bottom-up manner, the application of the refinement method together with the composition rule can guarantee their correctness.