Grain size distribution affects the distribution of melt in partially molten rocks, as melt is partitioned into the finer grained regions. As the creep strength of partially molten rocks depends on melt fraction, grain size distribution affects rheology in two ways: in the diffusion-creep regime the strength of the rock decreases with decreasing grain size; and by affecting the melt distribution, grain size distribution affects rock strength in the dislocation and the diffusion-creep regimes. To investigate this effect on partially molten dunites, we deformed synthetic aggregates of olivine + mid-ocean ridge basalt (MORB) with varying grain size distributions and melt fractions. We found that samples with bimodal grain size distributions did not obey the flow law derived for samples with a uniform grain size distribution. In the samples with bimodal grain size distributions the finer grained parts are enriched in melt, reducing the relative melt fraction in the coarser grained parts. The stress exponent is close to 1 for lower stresses for samples with non-uniform grain size distributions, while it is close to 3 for samples with a uniform grain size distribution. Electron backscatter diffraction (EBSD) analysis shows crystallographic preferred orientation (CPO) well developed in all samples. Microstructural investigation shows that partitioning of the liquid out of the coarser grained part of the aggregate into the finer grained part leads to a localization of the strain into the weaker, finer grained and melt-enriched part. By treating the two parts as mechanically different phases that deform at a uniform stress, we are able to approximate the rheological behaviour of the aggregates with bimodal grain size distribution.