Grain size control of any engineering metal is very important in the hot upsetting process. Generally, the grain size directly controls the mechanical properties and performance of the material. Al-B4C composite finds extensive applications in nuclear industries, defense, and electronic industries. Therefore, the aim of this work is to study the dynamic recrystallization (DRX) behavior of Al-4 wt % B4C composite during the hot upsetting test. Experimental work was performed on sintered Al-4 wt % B4C preforms at various initial relative density (IRD) values of 80%, 85%, and 90%, and over the temperature range of 300-500 C and strain rates range of 0.1-0.3 s1. The DRXed grain size of Al-4 wt % B4C preforms for IRDes, and temperatures and strain rates were evaluated by using an optical microscope. The activation energy (Q) and Zener-Hollomon parameter of sintered Al-4 wt % B4C preforms were calculated for various deformation conditions and IRDes. The mathematical models of DRX were developed as a function of Zener-Hollomon parameter for various IRDes to predict the DRXed grain size. It was found that the DRXed grain size decreases with increasing Zener-Hollomon parameter. Verification tests were done between the measured and predicted DRXed grain size for various IRDes, and absolute and mean absolute error was found to be 9.92% and 8.58%, respectively. Copyright © 2018 by ASME.