The effect of addition of a chain extender (ADR) creating branches on nylon 6 (PA6) by reactive extrusion on the isothermal crystallization behavior is studied. The effect of addition of small amounts of polyketone (PK) capable of forming strong hydrogen bonds with PA6 chains as an additive in reactive extrusion on the isothermal crystallization behavior is also studied. Addition of chain extender to PA6 results in a decrease in the degree of crystallization and the crystallization rate due to the formation of branches, and the increase in the chain extender content or reaction temperature speeds up the reaction forming branches to result in a greater decrease in the degree of crystallization and the crystallization rate. Addition of PK to PA6 results in a decrease in the degree of crystallization and the crystallization rate, and the degree of decrease is dependent on the amount of added PK. Addition of PK along with the chain extender results in a greater decrease in the degree of crystallization and the crystallization rate due to the additional formation of hydrogen bonds between branched PA6 and PK chains.