This paper studies inter-annual variations of 6.5-Day Waves (6.5DWs) observed at altitudes 20?110 km between 52°S?52°N latitudes during March 2002?January 2021, and how these variations were related to the equatorial stratospheric Quasi-Biennial Oscillation (QBO). Temperature amplitudes of the 6.5DWs are calculated based on SABER/TIMED observations. QBO zonal winds are obtained from an ERA5 reanalysis dataset. QBO phases are derived using an Empirical Orthogonal Functions (EOF) method. Wavelet analysis of the observed 6.5DW variations demonstrates obvious spectral maximums around 28?38 months at 32°N?52°N, and around 26?30 months at 32°S?52°S. In the Northern Hemisphere, peak periods lengthened poleward; in the Southern Hemisphere, however, they were unchanged with latitude. Residual 6.5DWs amplitudes have been determined by removing composite amplitudes from 6.5DWs amplitudes. Comparisons between QBO and monthly maximum residual 6.5DWs amplitudes (AMmax) show clear correlations between the QBO and 6.5DWs in both hemispheres, but the observed relationship is stronger in the NH. When AMmax were large in the NH, the mean QBO profile was easterly at all levels from 70 to 5 hPa; when the AMmax were weak, the mean QBO wind was weak westerly below 30 hPa. Linear Pearson correlation coefficients between QBO phases and AMmax show large positive values at 60?110 km between 20°N?52°N in April and around 64 km at 24°S in February, and large negative values from 80 to 110 km between 20°N?50°N in August and at 96?106 km between 20°S?44°S in February. These results indicate quantitative correlations between QBO and 6.5DWs and provide credible evidences for further studies of QBO modulations on long-term variations of 6.5DWs.