The present paper analyzes the optimal mass flow rate for steady, laminar, fully developed, forced convection in a helical coiled tube with fixed size and constant wall heat flux by the thermodynamic second law based on the minimal entropy generation principle. Two working fluids, including air and water, are considered. The entropy generation analysis covers a coil curvature ratio (delta) range of 0.01-0.3, two dimensionless duty parameters related to fluid properties, wall heat flux and coiled tube size, alpha(1) range of 0.01-3.0 and alpha(2) range of 0.1 x 10(-6)-1.2 x 10(-6). The optimal mass flow rate, denoted by a dimensionless parameter, beta(opt), for cases with various combinations of the design parameters is given in the present paper. In addition, a correlation equation for beta(opt) as a function of alpha(1), alpha(2) and delta is proposed through a least square error analysis. For a thermal system composed of helical coiled tubes with fixed wall heat flux and tube size, the optimal mass flow rate beta(opt) should be selected so that the system can have the least irreversibility and the best exergy utilization. (c) 2006 Published by Elsevier Ltd.