The $alpha$ to $eta$ phase transformation of $alpha$-Si3N4 whisker and related microstructural changes have been investigated. When only $alpha$-Si3N4 whisker was heat treated in the range 1650~175$0^{circ}C$, the $alpha$ to $eta$ phase transformation occured. In this case, it eas suggested that the oxygen content in $alpha$-Si3N4 whisker affected the transformation behavior. Although $alpha$-Si3N4 whisker with Si was heat treated under the same condition, however, the variation of $eta$- fraction had a similar tendency with heat treating time. Therfore, it was considered that the oxygen content in $alpha$-Si3N4 whisker affected the transformation behavior dominently rather than the content of added Si. The added $eta$ phase did not affect the transformation behaviro of $alpha$-Si3N4 whisker.