Abstract: The corkscrew oscillation of beam centroid in the linac makes the increment not only of the effective spot-size of the electron beam extracted from an accelerator but also of the normalized emittance of the beam. The expressions of the emittance growth due to the corkscrew oscillation induced by two sources are analytically given in this paper. The sources are the tilted beam injection and the statistical misalignments of the solenoidal focusing.