Photonic crystal nanobeam microcavities based on slot and hollow-core waveguides were proposed and analyzed. Three-dimensional finite-difference time-domain simulations show that both an ultrasmall modal volume (V) and a high quality (Q) factor can be obtained simultaneously in hollow-core-nanobeam mirocavities (HCNMs) due to the strong confinement of fields in the two lateral directions. For a $6.5\text{-}\mathrm{\mu m}$ -long HCNM, the Q factor and V are on the order of ${10}^5$ and ${10}^{-2}(\lambda /2n{)}^3$ , respectively. With compactness, lower fabrication requirements as well as ultrahigh $Q/V$ , the proposed microcavities would be very promising in a variety of applications.