The relationship between the complex blood flow inside the human heart and the cardiac pumping function has received attention in recent years. We previously proposed a method for high-frame-rate imaging of echoes from blood particles with diverging beam transmission in order to visualize the flow direction in the human heart. In the present study, the velocity vector estimator with speckle-tracking technique using the inter-frame movement of blood echoes acquired at a very high-frame-rate was evaluated. Furthermore, the visualization of streamline was demonstrated by the 4th-order Runge-Kutta method with the obtained blood velocity vector. The accuracy of velocity vector estimation by the speckle-tracking technique suffers from low signal-to-noise ratios of echoes from tiny echo sources (blood particles) and low acoustic output of diverging beam. Hence, in this study, two-dimensional correlation functions used for the speckle-tracking technique were temporally averaged for a stable estimation of velocity vectors. In the steady flow experiment, it was confirmed that the estimation accuracy was improved by averaging of correlation functions even during 2 ms. The echo data was acquired in in vivo measurement of a 27-year-old healthy male at the ultrahigh-frame-rate of 6250 Hz with the single transmission of a non-steered diverging beam per frame. The fluxes flowing into and out of the cardiac cavity were visualized by blood flow vectors estimated by the speckle-tracking technique with averaging of correlation functions. In mid diastole, the vortex-like flow appeared in the distribution of velocity vectors. The blood flow toward the apex side of the heart was visualized by the obtained streamline in rapid filling phase.