A statistical mechanics of the velocity and magnetic fields is formulated for an active region plasma. The plasma subjected to the conservation laws emerges in a most probable state which is described by an equilibrium distribution function containing a lagrange multiplier for every invariant of the system. The lagrange multipliers are determined by demanding that the measured expectation values of the invariants be reproduced. For a numerical exercise, we have assumed some probable values for these invariants. The total energy of a coronal loop is estimated from energy balance considerations. Doppler widths of the UV and EUV lines excited in the coronal loop plasma give a measure of the root-mean-square velocities. Measurements of magnetic helicity are not available for the solar corona.