Abstract: A screw rotor assembly having nonsymmetrical tooth profiles and used in a screw-type rotary compressor or expander. The tooth profile of the female rotor is formed such that a line (H.sub.2 -A.sub.2) is formed by a generated curve of a point A.sub.1 of the male rotor; a line (A.sub.2 -B.sub.2) is formed by a circular arc having a point O.sub.7 as the center of the arc and a radius R.sub.7 ; a curve (B.sub.2 -C'.sub.2) is formed by an envelope developed by a circular arc (B.sub.1 -C.sub.1) of the male rotor; a portion between points D'.sub.2 and E.sub.2 is formed by a circular arc having a point O.sub.1 as the center of the arc and a radius R.sub.1 ; a line (C'-D') is formed by a line smoothly connecting the curves (B.sub.2 -C'.sub.2) and (D'.sub.2 -E.sub.2); a curve (E.sub.2 -F.sub.2) is formed by a circular arc having a point O.sub.2 as the center of the arc and a radius R.sub.2 ; and a curve (F.sub.2 -G.sub.2) is formed by a circular arc having a point O.sub.8 as the center of the arc and a radius R.sub.8.