Supraventricular tachyarrhythmia (SVTA) which occurs on the atria placed above the ventricles is the most frequently encountered cardiac arrhythmias. Heart rate variability (HRV) is an important indicator for diagnosis and classification of cardiac disturbances. Besides, computer based analytical tools which are proposed as new methods and algorithms on HRV produce effective parameters for diagnosis of heart disease. The wavelet packet transform (WPT) is an efficient tool for analysis of non-stationary signals like HRV. This study presents critical frequency ranges for HRV analysis in SVTA patients. Sub-frequency regions are determined and analyzed in HRV data obtained from 78 half-hour ECG recordings in MIT-BIH (Massachusetts Institute of Technology-Beth Israel Hospital) supraventricular arrhythmia database. Each data is decomposed into sub-frequency regions using 9 levels wavelet Packet transform and these regions are analyzed and evaluated for each SVTA recordings. The results of this study show that especially the mean energy values of sub-frequency regions in very low frequency (VLF) band for each patient are higher than others included low frequency (LF) and high frequency (HF) bands. In addition, the mean energy values of the regions in LF band are higher than in HF bands as usual. 0.00390625 - 0.03125 Hz sub-frequency range in VLF band, 0.039625-0.0625 Hz and 0.093751-0.10938 sub-frequency ranges in LF band have higher energy values than other sub-frequency regions. However, the sub-frequency regions of HF band reflect the heterogeneous energy distribution.