Abstract: Monosymmetry section constants are key constants for calculating the critical moment of thin-walled members with flexural-torsiaonl buckling. The general integral formulae for monosymmetry section constants, β_x and β_y, are derived in the centroid axis coordinate system. The general summation formulae of β_x and β_y are derived based on the general integral formulae. Accurate and approximate formulae of β_x and/or β_y for some common cross-section are obtained from the general summation formula. The accuracy of those β_x and/or β_y formulae in China current code GB 50017-2003 and GB 50018-2002, 2013 AISI design manual and SSRC Guide and AS/NZS 4600: 2005 etc., are judged according to the proposed accurate formulae. The results shown that the general summation formulae of β_x and β_y are simple, reliable and useful.