A mathematical model is developed to investigate the nonlinear, isothermal steady-state free convection boundary layer flow in an incompressible third-grade viscoelastic micropolar fluid from a vertical isothermal cone. The non-Newtonian fluid exhibits both viscoelastic and micro-structural characteristics and is representative of certain polymers. The transformed conservation equations for mass, linear momentum, angular momentum and heat are solved numerically subject to realistic boundary conditions using the second-order accurate implicit finite-difference Keller-box method. The numerical code is validated with previous studies and also with a Nakamura tridiagonal method. Detailed interpretation of the computations is included. The present simulations are of interest in chemical engineering systems and solvent and low density polymer materials processing. © 2018, Shiraz University.