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Since the hot forming of TC4 alloy after diffusion bonding has extensive applications in aerospace and medical industries, it is practically significant to explore the influence of diffusion bonding on the alloy's hot formability and identify the optimal forming parameters. Therefore, dual-stage diffusion bonding of aviation grade TC4 is carried out firstly at 900 °C for 1 hr and then at 930 °C for 2 hr under 2 MPa normal pressure in 5.0 x 10-3 Pa vacuum atmosphere. Another block of the as-received alloy is subjected to the same thermal loading as the diffusion bonding process. The diffusion-bonded and heat-treated alloys are then subjected to uniaxial tensile tests at 750 – 900 °C and 0.0001 – 0.1 s-1. Based on the tensile test data, the constitutive and dynamic material models are developed to investigate diffusion bonding effects on the alloy's hot forming behavior and identify optimal forming conditions. The developed constitutive model showed good predictability. The apparent activation energy of diffusion bonded (440 – 510 kJ • mole-1) and heat-treated (400 kJ • mole-1) alloys indicated that dynamic recrystallization and recovery are the primary deformation mechanisms. The processing maps revealed that diffusion bonding expanded the instability domain and lowered the dissipation efficiency, limiting the safe working conditions of the alloy. The analysis of deformed microstructure validated the findings of processing maps. The optimal processing conditions of 800 °C/ 0.0001 s-1 and 850 °C/ 0.0001 s-1 are discovered for diffusion-bonded and heat-treated alloys, respectively.