Pexa-Vec (JX-594) is a specific cancer-targeted oncolytic and immunotherapeutic vaccinia virus. The purpose of this study was to develop methods to maximize the stability of Pexa-Vec. In short-term instability testing, viral activity was rapidly decreased both at 4°C and at room temperature (RT), but it was completely restored after sonication followed by vortex. Long-term stability testing of Pexa-Vec in the following liquid formulations was performed: (A) 30 mM Tris/pH 7.6, (B) 30 mM Tris/pH 8.6,(C) 30 mM Tris/pH 7.6, 150 mM NaCl, 15% sucrose, (D) 30 mM Tris/pH 7.6, 15% sucrose, and (E) 30 mM Tris/pH 8.6, 15% sucrose. Viral activity decreased less than 2 log10 at 4°C, and RT was observed in 3 days in B, while viral activity was not decreased even after 4–8 weeks at 4°C and at 1 week in RT in A, suggesting that neutral pH may be essential to maintain virus stability. The addition of 15% sucrose into A (D) significantly increased viral stability at -20°C, 4°C, or RT, and it was also observed at pH 8.6 (E). The addition of 150 mM NaCl into D (C) significantly increased viral stability in addition to the sucrose effect at 4°C or RT. Accordingly, the viral activity in formulation C was maintained for 1.5 years at 4°C, and for 1-2 weeks in RT. In conclusion, we propose that formulation C can provide the most adequate condition for the proper storage of vaccinia oncolytic virus.