In this work, the influence of the vertical tube diameter on the performance of the heat transfer is conducted in a gas-solid fluidized bed of 0.14 m inside diameter. The heat transfer coefficient is measured using an advanced fast-response heat transfer probe. Two tube diameters of (0.0254 and 0.0127 m) were used to study the reliance of the heat transfer coefficient of the immersed vertical tubes on the tube diameter, using glass beads solid particles of 365 µm average size and 2500 kg/m3 solid density (Geldart B particles), with a static bed height of 0.35 m. The experiments were conducted at different superficial gas velocities, axial heights (H/D), and three radial positions (r/R). It was found for all used operating conditions and measurement positions inside the bed that the local heat transfer coefficient is increased with an increase in the vertical tube diameter such that an enhancement in the heat transfer coefficient occurred when the immersed tube diameter increased from 0.0127 to 0.0254 m. The current common correlations available in the literature do not predict our experimental results well. Therefore, a new regression correlation was predicted using JMP®12 statistical software based on relevant dimensionless groups, with a good mean relative deviation value of 4.59% between the experimental and predicted data.