Pipe element coupler and coupling method thereof

A pipe element coupler includes a plurality of coupling members, a gasket disposed in the coupling members and a fastening element. Each of the coupling members includes two engaging keys at two coupling end portions thereof each having an engaging surface that has varying radiuses of curvature. The fastening element is coupled at the coupling members, wherein the pipe element coupler is adapted for being preassembled on the two pipe elements to align the engaging keys with the corresponding grooves of the pipe elements. When the fastening element is operated to fasten the coupling members together, each of the engaging surfaces is deformed to change the varying radiuses of curvature, so as to substantially match a radius of curvature of the groove circumference at the bottom of the groove of the pipe element.