Boussineśq and non-Boussineśq fluid with thermo-physical property variation have been investigated for a vertical fin array. Computations are executed for the range of parameter such as Grashof number 1.86 × 105 and 4.42 × 105, non-dimensional S* = 0.2, 0.3, and 0.5, and non-dimensional clearance C* = 0.10, 0.15, and 0.40. The axial development of various fluid flow quantities, such as the pressure defect (P*), local Nusselt number (Nul), and the bulk temperature of the fluid (θb) has been presented for each of the property combinations. Nul is observed to have reduced by 70% near the exit from the Boussineśq fluid with fixed viscosity and thermal conductivity to the non-Boussineśq fluid with the variable property. Furthermore, the overall Nusselt number (Nu) at S* = 0.2 in an isothermal vertical fin array for an increase in Gr from 1.86 × 105 to 4.42 × 105 with different combination of properties such as Boussineśq fluid with fixed viscosity and thermal conductivity (ρb μc kc), Boussineśq fluid with variable viscosity and thermal conductivity (ρb μv kv), non-Boussineśq fluid with fixed viscosity and thermal conductivity (ρv μc kc), and non-Boussineśq fluid with variable viscosity and thermal conductivity (ρv μv kv) are observed to have an increase of 138%, 148%, 150%, and 160%, respectively.