Abstract: A process for forming an ultra-high molecular weight nylon 6 by polymerizing .epsilon.-caprolactam, which comprises heating substantially anhydrous .epsilon.-caprolactam at a temperature of from 100.degree. to 200.degree. C. in the presence of (a) a main catalyst selected from a Grignard compound and a halogenated magnesium lactamate, (b) a co-catalyst selected from an acyl lactam compound and an isocyanic acid ester and (c) at least one activator selected from the group consisting of amides, sulfoxides, thioethers and phosphoric esters. According to this inventive process, an ultra-high molecular weight nylon 6 can be produced within a short period of time, making use of industrially advantageous compounds which are stable in air, hardly dangerous and inexpensive. Being free from cross-linkings, the ultra-high molecular weight nylon 6 obtained by the process of this invention is soluble in various solvents and is useful as a starting material for the production of high strength and high modulus materials.