Based on the noise-replacement paradigm, recent studies showed that vowels carried more perceptional information for sentence intelligibility than consonants. Considering that vowels contain many important acoustic cues for speech perception, this study further assessed the effect of spectral degradation to the intelligibility of Mandarin vowel sentences. Mandarin sentences were processed to generate three types of spectrally degraded [i.e., fundamental frequency (F0) flattened, sinewave synthesized, and noise-vocoded] stimuli. Noise-replacement paradigm was implemented to preserve different amounts of vowel centers and replace the rest with noise. Listening experiments showed that flattening F0 had a minimal effect on the intelligibility of Mandarin vowel sentences, and the harmonic structure within vowels accounted more for the intelligibility of Mandarin vowel sentences. While deleting vowel edges had little influence on the intelligibility of the unprocessed vowel sentences, it had a significantly negative effect on the intelligibility of vowel sentences with spectral degradation.