Compositions of natural olivine tholeiites were simplified in the system CaO-MgO-Al2O3-SiO2, and approximately eutectic composition in the system diopside(Di)-forsterite(Fo)-anorthite(An) (Di:Fo:An=49.0:7.5:43.5 wt%) was chosen as the model basalt composition. High-temperature drop calorimetry was performed at 1405-1676 K for the samples with the model basalt composition consisting of the mixture of synthetic diopside, forsterite and anorthite. The heat of fusion of the model basalt was obtained to be 506¿38 J/g from the difference between the heat content of liquid and that of the mineral mixture at 1543 K, which was the approximated eutectic temperature. This heat of fusion shows almost zero enthalpy of mixing in this system. Taking account of the pressure, temperature and composition effects on enthalpy, the heat of fusion of 490-560 J/g is estimated for generation of MORB (mid-ocean ridge basalt) in the upper mantle. ¿ American Geophysical Union 1995