This paper presents the results of performance evaluation of silicone treatment technique which was developed for the insulation rehabilitation of water tree aged XLPE power cables. We treated the water tree aged 325 [$ extrm{mm}^2$] CN/CV cables with silicone, and then analyzed the degree of insulation rehabilitation as a function of time. AC breakdown test was conducted to evaluate insulation rehabilitation. The diagnosis test using relaxation current measurement and the characteristic analysis of insulation were also performed to estimate silicone treated cable. AC breakdown strength of silicone treated cable for one year was increased, resulting from the chemical reaction between silicone fluid and water. This experiment showed that the silicone treatment technique was effective for insulation rehabilitation of the water tree aged cables.