Moire patterns observed in scanning tunneling microscopy of lattice-mismatched systems are studied theoretically. It is found that there are two cases showing the moire patterns. One is explained by only the electronic-structure effect. In the other case, the effect of the lattice displacement is important. As prototypes of the two cases, scanning tunneling microscopy images of monolayer-graphite and transition-metal-dichalcogenide surfaces are investigated by numerical calculations.