Recent experiments on the fracture strength of zinc oxide (ZnO) ceramics showed that the size of specimens has almost no influence on their mean strength although strength data of a set of nominally identical specimens are still found to scatter. It is suspected that the complex nature of defect–microstructure interaction and the high density of porosity in ZnO ceramics are possible reasons for this insensitivity to strength scaling. In the paper, numerical results obtained by finite element analysis show that the fracture strength of ZnO is more influenced by the pore/grain-size interaction than only by the size of a pore or its shape. As a consequence, the pore/grain-size interaction will increase the fracture probability of small pores and lead to a homogenisation of critical flaw sizes. Furthermore, the high degree of porosity, especially the heterogeneous distribution and clustering of pores, could favour further homogenisation of critical crack sizes. This implies that the fracture strength of ZnO is insensitive to the size of specimens as corroborated by experiments. Finally a simple statistical explanation is given.