When the molten state of a semicrystalline polymer is subjected to sufficiently intense flow before crystallization, the crystallization kinetics are accelerated and the crystalline superstructure is transformed from spherulites to smaller anisotropic structures. In this study, flow-induced crystallization (FIC) of polyamide 66 (PA 66) was investigated using rheology and polarized optical microscopy. After an interval of shear flow at 270 degrees C, above the melting temperature (T-m = 264 degrees C) and below the equilibrium melting temperature, small-amplitude oscillatory shear time sweeps at 245 degrees C were used to monitor FIC kinetics. As specific work was imposed on a PA 66 melt at 270 degrees C from 10 Pa to 40 kPa, the onset of crystallization at 245 degrees C did not change. Above the critical work of 40 kPa up to 100 MPa, the onset of crystallization at 245 degrees C was progressively shifted from 628 to 26 s, as the applied specific work was increased. For quantitative analysis of the acceleration, the Avrami equation was used with Pogodina's storage modulus normalization method, revealing the transition of Avrami exponent from similar to 3 to similar to 2 at the critical specific work of similar to 40 kPa. Strong FIC acceleration was observed after the transition. After applying very low shear rates, large spherulites were observed without cylindrites, while a mixture of small spherulites and large anisotropic cylindrites was seen after applying a shear rate of 10 s(-1).