In this paper, band structures and defect modes of two-dimensional magnonic crystals with line defect are calculated by using the plane-wave expansion method under supercell approximation. The obtained results show that line defect modes can be produced in the band gap by introducing the line defect bodies into the two-dimensional magnonic crystal. The presence of the line defect modes leads to the spin waves propagating along the direction of the line defects in these structures. Utilizing the wave-guide characteristic of line defect modes, two-dimensional magnonic crystals can be used as the materials for fabricating spin-wave waveguides.