Detection of an unknown word or non-vocabulary word uttered by the user is necessary in realizing an effective spoken language user-interface. This paper describes the evaluation of an unknown word processing method for a sub word unit based spoken word recognizer. We have assessed the relationship between the word recognition accuracy of a system and the detection rate of unknown words both by simulation and by experiment of the unknown word processing method. We have seen that the resultant detection accuracies using the unknown word processing are significantly influenced by the original word recognition accuracy.