We report a theoretical and experimental study of a channel drop filter with two cascaded point-defects between two line-defects in a two-dimensional photonic-crystal slab. Using coupled-mode analysis and a three-dimensional finite-difference time-domain method, we design a filter to engineer the line shape of the drop spectrum. A flat-top and sharp roll-off response is theoretically and experimentally achieved by the designed and fabricated filters. Furthermore, we theoretically demonstrate that drop efficiency is increased dramatically, up to 93%, by introducing hetero-photonic-crystals. We also describe a method to modify the bandwidth of the spectrum.