One-dimensional (1-D) photonic crystal (PhC) nanobeam cavity based on parabolic-shaped width and tapered holes was proposed and analyzed. Finite-difference time-domain simulations show that both an ultrasmall modal volume (V～ 0.04(λ /n)3) and ultrahigh quality factor (Q～ 1.8× 107) can be obtained in PT PhC nanobeam cavity. In addition, the PT PhC nanobeam have one order lower V comparing to parabolic and tapered nanobeam. With compactness, lower fabrication requirements to achieve ultrahigh Q/V, the proposed nanobeam could be versatile platforms of interest for optical communications, optomechanics, cavity QED, etc.