We measure the radial velocity curve of the eclipsing detached white dwarf binary NLTT 11748. The primary exhibits velocity variations with a semi-amplitude of 273 km s-1 and an orbital period of 5.641 hr. We do not detect any spectral features from the secondary star or any spectral changes during the secondary eclipse. We use our composite spectrum to constrain the temperature and surface gravity of the primary to be T eff = 8690 ± 140 K and log g = 6.54 ± 0.05, which correspond to a mass of 0.18 M sun. For an inclination angle of 89fdg9 derived from the eclipse modeling, the mass function requires a 0.76 M sun companion. The merger time for the system is 7.2 Gyr. However, due to the extreme mass ratio of 0.24, the binary will most likely create an AM CVn system instead of a merger. Based on observations obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.