The objective of this present work is to study the effect of initial relative density (IRD) on microstructure evolution of sintered Al–4wt.%B4C preforms during hot compression tests with different temperatures and strain rates. The hot compression tests of sintered Al–4wt.%B4C preforms were performed in a hydraulic press for different range of temperature (300–500) °C, strain rate (0.1–0.3 s−1), IRD (80–90)%, and deformation degree (10–50)%. The microstructure of sintered Al–4wt.%B4C preforms was characterized by optical microscope, and the average dynamic recrystallization grain size was calculated by intercept line method. It is found that the average grain size of sintered Al–4wt.%B4C deformed preforms is increased with increasing deformation temperature and IRD, whereas it decreased with increasing the strain rate and deformation degree. On the other hand, the average grain size of sintered Al–4wt.%B4C deformed preforms is dependent on the Zener–Hollomon (Z) parameter. © 2018, Springer Science+Business Media, LLC, part of Springer Nature and ASM International.