The effect of B2O3 addition on the electrical and microstructural properties of Ni0.5Co0.5ZnxMn2 - xO4 (where x = 0.15 and 0.3) negative temperature coefficient (NTC) thermistors was investigated. Ni0.5Co0.5Zn0.15Mn1.85O4 and Ni0.5Co0.5Zn0.3Mn1.7O4 samples were calcined at 900 degrees C and then sintered at 1100, 1200 and 1300 degrees C. Ni0.5Co0.5Zn0.15Mn1.85O4 + 0.1 mol B2O3 and Ni0.5Co0.5Zn0.3Mn1.7O4 + 0.1 mol B2O3 added samples were sintered at 900, 1000 and 1100 degrees C without applying calcination. The B2O3-doped samples sintered at 1100 degrees C without calcination gave rise to very similar crystal structure with un-doped sample. All samples sintered at 1100 degrees C consisted of a major cubic spinel phase, a minor tetragonal spinel and a minor NiO-rich phase. In addition, no peaks related to B2O3 or ZnO additions were observed. This result demonstrated that the B ions and Zn ions dissolved into the lattice. The bulk density and grain size of B2O3-doped samples without applying calcination were similar to calcined un-doped samples when all the samples were sintered at 1100 degrees C. It was also observed that the increase in Zn content had minimal effect on the bulk density and grain size of samples. The electrical resistivity and material constant for Ni0.5Co0.5Zn0.3Mn1.7O4 sample were 2475 omega cm and 3660 K and for Ni0.5Co0.5Zn0.3Mn1.7O4 + 0.1 mol B2O3 sample were 1570 omega cm and 3725 K when the samples were sintered at 1100 degrees C. NTC thermistor having lower electrical resistivity and higher material constant can be achieved with the addition of B2O3 without applying calcination.