The effects of hydrostatic pressure on the transition temperatures Tc of NH4IO3 and its solid solutions Kx(NH4)1-xIO3 have been investigated by means of differential thermal analysis (DTA) up to 10 kbar. The experimental results of Kx(NH4)1-xIO3 solid solutions indicates that the dTc/dP values are independent of the concentration of K+, although Tc decreased with increasing concentration. This appears to imply that the IO3- ions elongation under pressure will play a more important role in the pressure effect on the rising of Tc.