The magnetic behavior of 100 nm period arrays of Ni nanowires embedded in a highly ordered alumina pore matrix were characterized by magnetometry and magnetic force microscopy. Reducing the diameter of the nanowires from 55 to 30 nm while keeping the interwire distance constant leads to increasing coercive fields from 600 to 1200 Oe and to increasing remanence from 30% to 100% . The domain structure of the arrays exhibits in the demagnetized state a labyrith-like pattern. These results show that stray field interactions of single domain nanowires are entirely dependent on the nanowire diameter.