We report a study of aggregation behavior of polybenzimidazole (PBI) in polar aprotic solvents such as dimethylacetamide (DMAc). The photophysical studies of the PBI solution at various concentrations show concentration quenching and reveal that aggregated structures are formed when the polymer concentration is increased. The decay profiles obtained from time-resolved fluorescence study for low (0.00154 g/dL) and high (0.154 g/dL) concentrations of PBI in DMAc solution fit into a triexponential decay, surprisingly high concentration shows a growth (negative pre-exponential factor) in the decay profile, providing a support for excimer formation. The excited-state life time for the aggregated/excimer structure is found to be 4.14 ns, longer than that for the free polymer chains for which the life time is 502 ps. The concentration dependence emission spectra attribute that the aggregation/excimer formation is an intermolecular process. An abrupt decrease of Huggins constant and reduced viscosity with increase in concentration indicate the conformational transition of polymer chains of PBI from compact coil to an extended helical rodlike structure. The NMR and viscosity studies demonstrate that the intra- and intermolecular interactions (interchain hydrogen bonding) play an important role for the conformational transition and aggregation process. Transmission electron microscope images support the conclusion drawn from other studies; show helical rods for high concentration and featureless morphology for low concentration. The circular dichroism spectrum is also in agreement with the helical characteristics of aggregated structure. The temperature-dependent NMR and viscosity studies show that the disruption of interchain hydrogen bonding with increasing temperature destabilizes the aggregated structure at higher temperature.