A poly-silicon layer of a thin film transistor (TFT) having an active channel region, wherein a probability P that a maximum number of a primary grain boundary exists on the active channel region is not 0.5, the probability obtained by the following equation: P = D - (N max - 1) . Gs DIVIDED Gs , where D = L.cos&thetas; +W.sin&thetas;, L is a channel length of the active channel region, W is a width of the active channel region, Nmax is the maximum number of the primary boundary existing on the active channel region, Gs is a grain size, and &thetas; is a tilt angle of the primary grain boundary and the display device having thereof.
