The objective of this work is to investigate the effects of the square edged circular orifice plate thickness and Reynolds number on the flow characteristics using finite volume method for both laminar and turbulent flows. Here, the orifice thickness/diameter ratio is varied over the range of 1/12less than or equal tot(*)less than or equal to1 and for the Reynolds number range of 0less than or equal toRe(o)less than or equal to2x10(5) keeping the orifice/pipe diameter ratio, beta=0.6, constant. Hence, consideration in this work is given to the effects of Reynolds number and orifice thickness on discharge coefficient, C-d=f(Re, t(*)). Flow was assumed to be two dimensional, viscous, incompressible and steady for both laminar and turbulent flow regimes. Velocity distributions, formation of separated flow regions, flow discharge coefficients and hence the response of the orifice meter geometry to known upstream flow conditions is investigated as a function of Reynolds number and dimensionless orifice thickness (t(*)). The discharge coefficient, C-d, values are more sensitive to Reynolds number in the case of smallest values of t*. Minimum variation of discharge coefficient with respect to Reo occurs at t(*)=1 in both laminar and turbulent flows. Consequently, having t(*)=1 causes less pressure drop and less variations in Cd values comparing with the results of t(*)=1/12.