The electrical and microstructural properties of negative temperature coefficient (NTC) thermistors Ni0.5Co0.5CuxMn2-xO4 (0 aecurrency sign x aecurrency sign 0.3) have been investigated with B2O3 addition. The samples were prepared by conventional ceramic processing techniques. The B2O3-free samples were calcinated at 900 A degrees C for 2 h and then were sintered at 1100 A degrees C for 5 h. The B2O3-added samples were sintered at 900 and 1100 A degrees C for 5 h without applying calcination. The cubic spinel phase was identified in all samples by the X-ray diffraction analysis. The addition of B2O3 and CuO play an important role by increasing the bulk density, improving the microstructure properties and enhancing the electrical characteristics of NTC thermistors even without calcination. The values of electrical resistivity at 25 A degrees C, material constant (B-25/85) and activation energy (E-a) of all samples were in the range of 28-6903 Omega cm, 3085-3797 K and 0.266-0.327 eV, respectively. This means that the electrical properties of NTC thermistors can be controlled to a wide range of values to meet various requirements by the addition of B2O3 and CuO.