Reliability is the level of performance success of a system or part of the system, to be able to provide better results over a certain period of time and under certain operating conditions. To be able to determine the level of reliability of a system, can be done by performing calculations and analysis of the success rate or operation of the system under review, at a certain period and then compare it with predefined standards. There are several parameters that can be used to determine the reliability level of the power distribution system by calculating the average number of system interruptions during the year or SAIFI, the average duration of system disturbance index for one year or SAIDI (System Average Drop Duration Index) and CAIDI (Customer Disruption Duration Length Index). Based on the calculation and the analysis that has been done, it can be seen that the reliability value of Biru Substation 150 kV Bantul in the year 2015-2017 can be said not reliable for the value of SAIDI and its CAIDI in accordance with the standard SPLN No. 68-2: 1986 and IEEE Std 1366-2003. While the value of SAIFI on the 150 kV Substation of Bantul is only 2017 which can be said reliably according to SPLN. 68-2: 1986, Whereas according to IEEE std 1366-2003 standard can be said not reliable. The highest frequency and duration of shutdown occurred in 2016 with SAIFI value of 6.66 times / interruption / year, SAIDI value 93.98 hours / customer / year and CAIDI value 11.28 hours / year.