Long-term variations of the equatorial quasi-biennial oscillation (QBO) in the stratosphere are investigated using the reanalysis data and Singapore radiosonde data. A wavelet analysis with a Molret mother wavelet is used to obtain the QBO amplitude defined as 20-40 months period. The QBO components are also calculated with a Lanczos band-pass filter and the QBO amplitudes are defined as √2σ (σ: a root mean square of monthly time series) over consecutive three cycles assuming a monochromatic wave. Long-term trend and decadal variations are evidently analyzed in the QBO amplitude evolution. Composite picture, which is the difference between maximum and minimum phases in the QBO decadal variations reveal that the QBO decadal variations is not symmetric with respect to the equator but have significant interhemispheric differences depending on altitudes.