Although it is thought that orientation selectivity first emerges in the primary visual cortex, several studies have reported that neurons in the cat lateral geniculate nucleus (LGN) are sensitive to stimulus orientation, especially for high spatial frequency (SF) stimuli. To understand how this orientation sensitivity emerges, we investigated the spatiotemporal structures of linear receptive fields (RFs) of LGN neurons. Orientation tunings at several SFs were measured using sinusoidal drifting grating stimuli. Fine spatiotemporal structures of the linear RFs were measured using a reverse correlation technique and two-dimensional dynamic Gaussian white noise stimuli. A non-linear response modulation function was estimated by comparing measured responses with responses predicted from a linear RF structure. Although we found that a population of LGN neurons exhibited significantly elongated linear RF centers and that the angles of the long axes corresponded well to the preferred orientations, the orientation tunings predicted from the linear RFs were significantly broader than those measured. These results suggest that orientation-tuned non-linear response modulation induced by stimulation outside the classical RF contributes to the sharp orientation tuning seen in LGN neurons.