The self‐consistent T‐matrix solution envisaged by the effective‐medium approach [R. Zeller and P. H. Dederichs, Phys. Status Solidi B 55, 831 (1973)] has, in general, led to a considerable clarification of our understanding of the mechanical properties of a variety of disorder systems including polycrystals. Specifically, the relevant formulations have been developed for cubic polycrystals by Zeller and Dederichs, and for hexagonal, tetragonal, trigonal, and orthorhombic polycrystals by T. R. Middya and A. N. Basu [J. Appl. Phys. 59, 2368 (1986)]. The present work on monoclinic polycrystals is a sequel to our previous work. We have developed the complete set of equations within the framework of the effective medium theory which delivers in a self‐consistent manner the effective elastic constants of a monoclinic polycrystal in terms of those of the single crystal data. For comparison we have also evaluated the same quantities for each polycrystal by the computer simulation employing the velocity averaging process developed by T. R. Middya, A. N. Basu, and S. Sengupta [J. Appl. Phys. 57, 1844 (1985)]. Finally the results obtained by the above approaches for eight different polycrystals are compared with each other and also with other existing theoretical calculations.