<b>Analysis of 1 out of 2 Main Units and 2 out of 4 Subunits System</b> Introduction of redundancy is one of the well known methods by which the reliability of a system can be improved. A standby redundant system is one in which one operating unit is followed by spare units called standbys. On failure of operating unit, a standby unit is in working mode. In the present paper we investigate the probabilistic analysis of a two main unit and four subunit system. The system remain operative if one main unit and two subunit are in working mode. System is failed when the main unit fails. A failed system is replaced by new one. The failed sub unit are repairable. Reliability, Availability, Mean time to system failure, busy period analysis, Markov renewal process 555-562 Issue-2 Volume-2 Praveen Gupta | Ruchi Yadav