The polymorphic transition ( gamma -> alpha conversion) in a single nylon 6 filament under stretching has been explored for the first time by using micro-FT-IR spectroscopy. The content of gamma-form deceases with straining while the amount of alpha-form gradually increases, suggesting gamma -> alpha a conversion. A two-step mechanism, that is, melting and recrystallization, seems pertinent for the gamma -> alpha conversion considering that the gamma-form shows somewhat reversible and the alpha-form keeps nearly intact upon unloading. Moreover, stress-induced gamma -> alpha conversion at large strain can be well correlated with the molecular orientation in the amorphous phase and thus a serial arrangement between the gamma crystals and amorphous phase along the stretching direction is proposed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 898-902, 2009