The objective of this work was to improve thermal and rheological properties of poly(lactic acid) (PLA), through a reactive extrusion process. PLA was reactively modified in the melt state by using a multifunctional co-agent (triallyl trimesate) in the presence of dicumyl peroxide. The viscosity, elasticity and melt strength of PLA increased substantially following reactive compounding. Significant strain hardening was achieved in modified PLA. The coagent-modified PLA was compared to a chain-extended PLA, produced by a using functionalized epoxy (Joncryl). The improvement in rheological properties of PLA modified by co-agent and Joncryl were comparable. A significant enhancement in the crystallinity of the coagent modified PLA, by 114 %, indicating a strong nucleation effect was noted, as compared to 39 % enhancement by that of Joncryl-modified PLA. Moreover a distinct crystallization peak appeared at 133°C. Furthermore, the rate of crystallization of co-agent modified PLA was significantly higher than the Joncryl/PLA formulation (t1/2 = 0.56 vs. 9.34 min), suggesting clearly a nucleating effect, even though no nucleating agent was added. Investigation of the crystallization kinetics of co-agent modified PLA using the Avrami model confirmed these findings.