Analyzed in this paper is the cascade spectrum of Langmuir waves, which are the high-frequency sideband of the parametric decay instability or oscillating two-stream instability excited by an O mode HF heating wave. It involves the decay of a Langmuir pump wave into a Langmuir sideband and an ion acoustic wave, which is heavily damped by ion Landau damping. Both resonant and nonresonant cascade processes are considered. The nonresonant cascade of Langmuir waves proceeds at the same location and is severely hampered by the frequency mismatch effect, because the decay wave is a driven ion mode oscillating at considerably lower frequency than that of the ion acoustic wave. In contrast, the resonant cascade, which takes place at different resonant locations to minimize the frequency mismatch effect, has to overcome the propagation loss of the mother Langmuir wave in each cascade step. The resonant cascade has a lower threshold, but the cascade lines spread over a range of altitude. The dominant factors, which determine the number of cascade lines in the radar-detected spectrum of HF enhanced plasma lines (HFPLs), include the ion to electron temperature ratio, Ti/Te, the background plasma inhomogeneity scale length, and the heating wave field intensity. The proposed process can be a reasonable basis for explaining the radar-measured cascade spectrum of Langmuir waves (i.e., HFPLs) in Arecibo and Tromso heating experiments.