At the same solid volume fraction (Phi) the relative viscosity (eta (r)) of a concentrated noncolloidal bidisperse suspension of hard spherical particles is lower than that of a monodisperse suspension. In this paper a semi-analytical viscosity model of noncolloidal bidisperse suspensions is derived using an integration method. In this model the random loose packing density obtained by computer simulation is taken as the limit of solid volume fraction Phi (m) which depends upon both the diameter ratio (lambda) of large to small particles and the volume fraction of large particles ( xi = Phi (l)/Phi). This model shows that at high solid volume fraction, Phi > 0.40, both lambda and xi significantly influence eta (r). For example, at Phi = 0.5, it predicts that for monodisperse suspensions eta (r) = 70, while for bidisperse suspensions (lambda = 2 and xi = 0.7) eta (r) = 40. Comparison shows that, at high solid volume fraction (0.4-0.5), the relative viscosity predicted by this model is in good agreement with that predicted by the work of Shapiro and Probstein (1992) and of Patlazhan (1993), but is higher than that predicted by the work of others.