This study presents the thermal behavior of two young turbulent spots merging into a longitudinal direction on an isothermal flat plate for the local Reynolds number between 6.1 x 10(4) and 1.3 x 10(5) in a low freestream turbulence water tunnel having a turbulent intensity of 1.16%. The two turbulent spots are generated by water injection through a 1-mm-diameter hole in the perpendicular direction of the mainstream flow with a dimensionless separating time (Delta tau) of 42.08, 84.16, and 126.24. Thermochromic liquid crystals are utilized mutually with an image processing technique to extract the spot characteristics qualitatively and quantitatively. The results demonstrate that the following turbulent spot directly causes an increase in the local Nusselt number and heat rate within the footprint of the merging spots. The relatively highest increase in this study occurs when Delta tau = 84.16. The average Nusselt number and effectiveness characterize differently in the intersection zone, non-intersection zone of the leading spot, and non-intersection zone of the following spot. The results confirm that turbulent spots under the boundary layer transition augment the heat transfer rate to the level of full turbulence by not only their spot maturity but also the merging mechanism. Finally, the heat transfer mechanism is discussed and the predictive formulas for the Nusselt number and heat flux of the longitudinal merging of turbulent spots for Delta tau from 0 to 126.24 are provided.