We propose an ultracompact channel drop filter (CDF) based on photonic crystal nanobeam cavities.The conditions for implementing such an ideal CDF are derived from the temporal coupled-mode equations goveing the operation of the CDF.By considering the intercoupling of the two involved nanobeam cavities,some ambiguities of the previous equivalent circuit model analysis are cleared up.Practical configurations on silicon-on-insulator (SOI) for the proposed CDF are suggested with a typical length less than 15μm.Finite difference time domain (FDTD) method calculations show that the proposed filter can achieve drop efficiency higher than 99％ without any reflection.Compared to the λ/4-shifted Bragg grating resonators based CDF,the proposed CDF is more compact,high-efficient and reflection-free.It is also easy to implement a low-power tunable filter due to the ultrahigh quality factor Q and ultrasmall modal volume V of the involved photonic crystal nanobeam cavities.