The probability theory of reliability has grown out of the demand of modern technology which says the probability of a system or unit giving satisfactory performance for a specific period unit specified operating condition when a system or unit does not perform satisfactorily, it said to have failed. In the study, the standby unit can work for a certain time period and come to downstate even without failure. It’s working due to some failure. This failure of the units causes the failure of the complete systems due to some unknown reasons. They studied the system and designed a model for its reliability. In this present study, we considered a system with two non-similar and non-identical units causes sudden failure due to unknown reason. This sudden failure can occur in the system, causes the total system failure. The model is analyzed various measures reliability of the system.