This paper aims to investigate theoretically the electric fields and the surface irregularity factors for stranded conductors in pressurized air and SF6 in co-axial cylinder configurations. The surface irregularity factor is defined as the ratio of the corona onset voltage of a stranded conductor to that of a smooth cylindrical conductor of the same outer diameter. This calls for the assessment of the electric field in the vicinity of the stressed conductor and the corona onset voltage. The investigated gap is a three dimensional field problem. To solve this problem, charge simulation technique (CST) is used, where the simulation charges for stressed and grounded conductors are helical of infinite length. The method of corona onset voltage calculation is based on the criterion developed for the formation of repetitive negative corona Trichel pulses. The effects of varying conductor radius, gas pressure, stranding ratio on the irregularity factor values are investigated. The calculated values of irregularity factor in air agree well with those calculated and measured before.