This paper presents a theoretical and experimental study of the effect of conducting particles on the onset voltage of a negative corona in air-insulated co-axial cylindrical configurations. The conducting particles are spheres or wires of varying dimensions. The particle is fixed either on the inner or the outer cylinder. The calculated onset voltage of the negative corona is based on the criterion developed for the formation of repetitive negative coronas, Trichel pulses. This calls first for an accurate calculation of the electric field in the vicinity of the particle where avalanches grow. The investigated gap in the presence of a particle is a three-dimensional field problem due to the asymmetrical position of the particle inside the gap. The three-dimensional electric field is calculated using the charge simulation technique with a new charge distribution. An experimental set-up is built up to measure the onset voltage of a negative corona initiated by particles in an air-insulated co-axial configuration to check the accuracy of the present calculation. The effect of varying field nonuniformity, particle shape, size and position on the onset voltage of the negative corona is investigated. The calculated onset voltage values agree well with those measured experimentally.