Gas pressure sintered silicon nitride based composites with 5 wt% $eta$-Si3N4 whiskers were prepared, and the variations depending on sintering additives and sintering temperature were studied. Sintering additives were 6 wt% Y2O3-1 wt% MgO(6Y1M), 6 wt%Y2O3-1 wt% Al2O3(6Y1A), 6 wt% Y2O3-1 wt% SiO2(6Y1S), and whiskers were unidirectionally oriented by a modified tape casting technique. Samples were fully densified by gas pressure sintering at 2148 K and 2273 K. As the sintering temperature increased, the size of large elongated grains was increased. Three point flexural strength of 6Y1M and 6Y1M samples was higher than that of 6Y1S sample, and the strength decreased as the sintering temperature increased. The indentation crack length became shorter for the sample sintered at higher temperature, and the difference between the cracks length parallel to and normal to the direction of whisker alignment was decreased. In case of cracks 45$^{circ}$off the whisker alignment direction, the crack length anisotropy disappeared.