Tangential oscillation of thermal stratification causes periodic changes of temperature at pipeline inner surface. Such periodic temperature changes are accompanied by thermal stress disturbance of pipes, resulting in thermal fatigue damage. At North China Electric Power University, STOLTS test facility has been designed and constructed to investigate the initiation mechanism of tangential oscillation of thermal stratification at a pipeline system. In this work, experiments have been performed at a pipeline elbow with different vertical temperature gradients and mass flow rates in the inlet flow. Results show the hot fluid accumulates intrados of the elbow from original top position of the pipeline, which indicates an angular shift of the thermal stratification duo to the effect of centrifugal force. Vertical temperature gradient and mass flow rate show an impact on distribution of flow temperature. Moreover, temperature fluctuations have been calculated based on data from thermocouples measurement data downstream of the elbow. An overlapped region has been observed between areas of high temperature and high temperature fluctuation downstream of the elbow. In addition, frequency analysis has been performed to the temperature data. No significant peak has been found in the PSD diagrams. It can be understood that vertical temperature difference in the inlet flow is not large enough to maintain the stability of the thermal stratification, so that the tangential oscillation of thermal stratification is not initiated downstream of the elbow. However, increase of the vertical temperature gradient or flow rate in the inlet flow leads to increase of temperature fluctuation near the pipe wall. It indicates turbulent mixing is enhanced at elbow within the thermal stratification flow. Such a high temperature fluctuation shows in a high potential risk of thermal fatigue.