River bank erosion has always been a challenging problem in Bangladesh. Conventional method of designing erosion protection structures are governed by the hydraulic loads resulting from currents and waves. In practice, toe protection elements are dumped into flowing water and settle somewhere on the river bed to form an apron. The present study has been undertaken to investigate experimentally the aspect of underwater threshold condition of toe protection elements. The experiments are conducted in the large tilting flume of the Hydraulics and River Engineering Laboratory of Water Resources Engineering Department, BUET. Cubeshaped Concrete Block (CC block) is considered as erosion resistant material. A geometrically similar undistorted scale factor of 20 has been selected to conduct the experiment. Hydraulic parameters and CC block are selected based on typical field condition and the experiment is designed according to the scale. Two different initial water level (set-up 1 and set-up 2) is deliberated for the same size of CC block to investigate the hydraulics of threshold characteristics. During experimentation various observations are made and the measured data are used to analyze various hydraulic features of threshold condition. Gradual smooth increment of flow is ensured to reach threshold condition. At this stage, water level is 0.212 m, section average velocity is 0.55 m/s and depth averaged velocity is 0.54 m/s for set-up 1. While for set-up 2, water level is 0.256 m, section average velocity is 0.54 m/s and depth averaged velocity is 0.53 m/s. Though the threshold velocity magnitude for setup 1 and set-up 2 are very close, the corresponding water level is quite different since initial depth of flow is different. It is expected that the results obtained here can be useful to develop predictive expression for estimating the threshold velocity of CC blocks. Scope of detailed analyses in this area of interest is in progress.