The COBE satellite includes two cryogenically cooled instruments, which are housed inside a 664 liter superfluid helium dewar to achieve an operating temperature of 1.5K. The dewar will provide a 14 month operating lifetime, during which the far infrared absolute spectrophotometer (FIRAS) and diffuse infrared background experiment (DIHBE) will conduct full sky surveys from a 900 km, sun-synchronous polar orbit. The spectrum of diffuse radiation will be characterized by these instruments over the wavelength region from 1 micron to 1 cm. Launch is planned for autumn 1987. The dewar is very similar in design and function to that used successfully as part of the Infrared Astronomy Satellite (IRAS). Some design changes, as compared to the IRAS dewar, have been made to meet STS safety and operations requirements, to accommodate the CUBE instrument package, to increase lifetime, and to improve functional reliability in several areas. Most noteable design changes are a 23 percent enlargening of the cryogen tank, de-letion of the aperture cover helium tank, and improved thermal radiation baffling to mini-mize the heat input through the area at the interface between the main dewar and the aperture cover. Dewar system testing is scheduled to start in summer 1985. The dewar design and design drivers, especially as related to changes from the IRAS design, are discussed along with thermal performance predictions resulting from computer modelling and observed performance of the IRAS dewar.