  The GSO projection in the twisted sector of orbifold background is sometimes
subtle and incompatible descriptions are found in literatures. Here, from the
equivalence of partition functions in NSR and GS formalisms, we give a simple
rule of GSO projection for the chiral rings of string theory in $\C^r/\Z_n$,
$r=1,2,3$. Necessary constructions of chiral rings are given by explicit mode
analysis.
