Lightning Arrester is the main equipment in the substation that functions to protect other electrical equipment from surge disturbances, both lightning surges and circuit surges. Lightning arresters have the advantage of protecting electrical equipment from short circuit disturbances and lightning strikes. When a lightning strike or short circuit occurs, the wave will flow through the conductor in the transmission system at a speed close to the speed of light. To ensure that the lightning arrester post isolator is still suitable for use, insulation resistance and leakage current tests are carried out. The standardization of the insulation resistance value refers to IEEE 43-2000 and SK DIR 0520, which is <1GΩ, while the specified standardization of LCM leakage current measurements is ≤90%. This research was conducted at PT. PLN (Persero) GISTET 500kV Duri Kosambi. The results of the insulation resistance measurement before replacement showed that the R phase had a value of 5 MΩ, the S phase 4.91 MΩ, and the T phase 26.2 MΩ. Compared with the minimum standard value of 11,250 MΩ and the standards of IEEE 43-2000 and SK DIR 0520, which are <1GΩ, the R and S phases are below the standard, indicating unfitness for operation. After the replacement, the insulation resistance values increased significantly to 173,000 MΩ (R phase), 178,000 MΩ (S phase), and 153,000 MΩ (T phase), all of which are far above the standard and indicate a safe and proper condition. For the leakage current before replacement, the R phase showed 66.67% and the S phase 74.67%, both in the "Monitoring" category (51%-80%), while the T phase 28.67% is in the "Good" category (0%-50%). After replacement, the leakage current values decreased to 8% (R phase), 5.8% (S phase), and 13.3% (Tphase), all of which were in the "Good" category (0%-50%), indicating improvement in the insulator condition