Condenser is a heat exchanger used to condense the steam from Low Pressure Turbine into water. The condenser vacuum is always maintained within a given range provided by the manufacturer because condenser vacuum greatly affects the value of efficiency of steam turbine. The problem in this plant was that the pipes in the condenser were dirty which shown by the declining percentage of condenser cleanliness and it caused the value of condenser vacuum approach atmospheric pressure that made the generating unit shut down. This study was to analyze the relationship among steam turbine efficiency and the percentage of condenser cleanliness to the values of condenser vacuum. This research method was to find the efficiency of steam turbine and the percentage of condenser cleanliness on February 01 2017, February 24 2017, March 20 2018, and March 23 2018 when the values of condenser vacuum and steam turbine actual load were -707.9 [mmHg gauge] and 215.7 [MW], -696.5 [mmHg gauge] and -2.52 [MW], -705.1 [mmHg gauge] and 211.14 [MW], and -703.8 [mmHg gauge] and 201.87 [MW] using Performance Calculation Formula from Mitsubishi Heavy Industry LTD. The results showed that the best efficiency of steam turbine was 30.97% when the condenser vacuum was -707.9 [mmHg], the steam turbine actual load was 215.7 [MW], and the percentage of condenser cleanliness was 61.01%. The lowest efficiency of steam turbine was -0.91% when the condenser vacuum was -696.5 [mmHg], the steam turbine actual load was -2.52 [MW], and the percentage of condenser cleanliness was 25.39%. the negative value of steam turbine actual load indicated that the generating unit shut down.