Nucleus is one of membrane-bound organelles and important for maintenance of genome DNA in eukaryotic cells. Nuclear size, required for nuclear function, is proportional to cell size, resulting in a constant ratio between nuclear and cellular volumes (N/C ratio). However, how this is achieved remains elusive. To understand molecular mechanisms of nuclear size control in eukaryotic cells, we studied N/C ratio mutants using fission yeast, a model organism of eukaryotic cell. We found that properly regulated nucleocytoplasmic transport and membrane growth are critical for appropriate nuclear size control in growing cells. This finding may contribute to understanding a cause of cancer, because cancer cells have enlarged nucleus.