Photocrosslinkable polymers such as gelatin methacrylate (GelMA) have various 3D bioprinting applications. These polymers crosslink upon exposure to UV irradiation with the existence of an appropriate photoinitiator. Two photoinitiators, Irgacure 2959 and lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) are commonly used. This study systematically investigates the effects of photoinitiator types on the cell viability, physical properties, and microstructure in 3D bioprinting of GelMA-based cellular constructs. The main conclusions are: (1) during the 3D bioprinting, the cell viability generally decreases as the photoinitiator concentration and printing time increase using both Irgacure 2959 and LAP. At the low photoinitiator concentrations (such as 0.3% and 0.5% (w/v)), the overall cell viability is good within the printing time of 60 min using both Irgacure 2959 and LAP. However, at the high photoinitiator concentrations (such as 0.7% and 0.9% (w/v)), the overall cell viability using LAP is much higher than that using Irgacure 2959 within the printing time of 60 min; (2) after the 3D bioprinting, the photoinitiator types, either Irgacure 2959 or LAP, have negligible effects on the post-printing cell viability after crosslinking; (3) after the 3D bioprinting, GelMA samples cured with Irgacure 2959 have slightly larger pore size, faster degradation rate, and greater swelling ratio compared to those cured with LAP; (4) 3D GelMA-based vascular-like constructs have been fabricated using dynamic optical projection stereolithography, and the measured dimensions have been compared with the designed dimensions showing good shape fidelity.