Many simple software errors are found in earlier software test phases.The ratio of complex errors to simple errors gradually increases with continual testing.This paper describes a software reliability model called the Error Complexity Model.In this model, errors are classified by error complexity which is a measure of error detectability.The number of remaining software errors is estimated from the ratio of complex to simple errors and the number of discovered errors.New criteria for error complexity classification are proposed.The model is evaluated and compared with existing models using actual error data.