Orthorhombic kalsilite (KAlSiO4) was prepared by solid-state reaction from K2CO3, Al2O3, and SiO2. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6 x 10(-5)degrees C-1 for the a-axis, 1.6 x 10(-5)degrees C-1 for the b-axis, 2.8 x 10(-5)degrees C-1 for the c-axis, and 2.0 x 10(-5)degrees C-1 for the average from room temperature to 1000 degrees C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li2CO3. The KAlSiO4 ceramic sintered at 1200 degrees C for 2 h with 5 wt % Li2CO3 had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2 x 10(-5)degrees C-1 from room temperature to 600 degrees C.