[en] We analyze the time series of oxygen line profiles (OVI 1031.93 Å and OVI 1037.61 Å) obtained from SUMER/SOHO on the solar south limb. We calculated Doppler shifts and consequently Doppler velocities in three heights 4″, 14″, and 24″ from the limb on a coronal hole region. Then, we performed wavelet analysis with Morlet wavelet transform to determine the periods of fundamental mode and its first harmonic mode. The calculated period ratios have departures from its canonical value of 2. The density stratification and magnetic twist are two main factors which may cause these departures.