For some 3d transition-metal monopnictides, their physical properties are strongly affected by interstitial 3d transition-metal atoms, such as Fe1+delta Sb, in which the electrical transport, magnetic properties and carrier concentrations have strong dependence on the composition of interstitial Fe (Shiomi et al. Phys. Rev. Lett. 108(5), 056601 2012), Therefore, it is important to study the influence of interstitial Co on the physical properties of Co1+delta Sb. Here, the magnetic and transport properties have been studied on Co1.05Sb single crystals. The magnetic and electrical transport measurements reveal that stoichiometric CoSb is a nonmagnetic metal. The interstitial Co in this compound exhibits a paramagnetic behaviour and weak antiferromagetic correlations. However, it has no effect on transport properties. A very low magnetoresistance (ae 0.2%) was observed under the magnetic field up to 14 T, which suggests that Co1+delta Sb could be a potential material of electronic devices for the application at a high magnetic field.