We theoretically and experimentally investigate the characteristics of a multichannel add/drop filter which utilizes in-plane hetero photonic crystals (IP-HPCs). The structure consists of an in-plane array of photonic crystals with different lattice-constants. Finite-difference time-domain calculations reveal that optimal performance in terms of wavelength resolution and efficiency can be kept almost constant at different wavelengths even though slab thickness is not changed. The multichannel add/drop device fabricated consists of seven photonic crystals, with a lattice parameter difference of 1.25 nm between neighboring regions. This device demonstrated wavelength spacing similar to 5 nm with almost constant Q factor and efficiency. It is also shown that the boundary between different PCs (the heterointerface), plays an important role not only in improving add/drop efficiency but also in performing directional filtering.