A new vehicle for a soft gelatin capsule with the basic formulation of PEG 400: water: medium chain triglyceride: polyoxyethylene (20) cetylether = 77: 10: 10: 3 was modified by partially replacing PEG 400 with PEG 6000 and added with vitamin K₂ as a model drug, and the effects of PEG 6000 on the stability of the vehicle and vitamin K₂ were studied for a period of 8 weeks at 4°C, room temperature and 40°C. For the vehicle containing 0.2% vitamin K₂ (a vitamin dose), temperature-dependent gel-sol transformation was prevented by replacing 2.0 or 3.0% of PEG 400 with PEG 6000, and the particle size distribution and appearance were maintained stable during 8 weeks of storage at each temperature tested. The vitamin K₂ was also found to be stable during the storage. Also for the vehicle containing 5.0% vitamin K₂ (a pharmacological dose), both of the vehicle and vitamin K₂, were found stable during 8 weeks of storage at every temperature. These results suggest that PEG 6000 plays an important role in maintaining the stability of vitamin K2 and the vehicle, preventing coagulation of particles, and this vehicle system could be used for preclinical and clinical trials.