Different additives’ influence on electric conductivity of ‘dielectric/conductor’-type composites (namely, polycrystalline $c$BN-based composites contained $c$BN as a dielectric phase and NbN as a conductor phase) is investigated. Investigated samples are obtained by the application of HPHT (high pressure-high temperature) sintering ($P$ = 7.7 GPa, $T$ = 2000°C). Electrical-resistance dependences on the temperature and applied voltage are measured for all samples. Obtained results’ analysis shows that all sintered samples have a semiconductor nature of conductivity. It is interesting that addition dielectrics ($Al_{2}O_{3}$, $Si_{3}N_{4}$) as well as semiconductor (SiC) leads to the electric-conductivity improve (drop in electrical resistance) for $c$BN–NbN composites in some degree (despite the lower electrical conductivity of these substances). Alumina whiskers’ addition to $c$BN–NbN composites leads to a more significant drop in electrical resistance compared to powder particles’ addition (from 1.85 to 0.72 Ohm$\cdot$cm for samples with whiskers (Al$_{2}$O$_{3}$w) and from 1.35 to 0.17 Ohm$\cdot$cm for samples with Al$_{2}$O$_{3}$ powder). Hence, as concluded, the sample electrical conductivity is affected by both the additive particles’ morphology and the grain boundaries’ state of the sintered polycrystalline ceramics.