Thermal equation of state (EoS) of synthetic lawsonite [CaAl2Si2O7(OH)(2)center dot H2O] has been established using insitu X-ray diffraction methods under high pressure and high temperature. Sodium chloride NaCl was used as the pressure standard in the experiments. The unit-cell volumes were measured up to 10 GPa and 973 K after the deviatric stress was released at high temperature. The P-V-T dataset was analyzed using a Birch-Mumaghan equation of state, yielding the room pressure volume V-0 = 674.2(2) angstrom(3) and the isothermal bulk modulus at room temperature K-0 = 129(2) GPa (K' set to 4). These values are comparable with the previous studies. When fitting the high temperature data, a second order temperature derivative of the bulk modulus was considered. Unlike Daneil et al. (1999) who reported a minimum value of bulk modulus at similar to 500 K, the bulk modulus decreases with increasing temperature at least up to 973 K. The dataset yields: V-0 = 674.3(4) angstrom(3), K-0 = 128.7(15) GPa, (partial derivative K-T/partial derivative(T))(P) = -0.047(8) GPa K-1, (partial derivative(KT)-K-2/partial derivative T-2)(P) = 0.028(6) x 10(-3) GPa K-1, alpha = 3.13(25) x 10(-5) K-1, assuming K' = 4. These data can be used to calculate the density and the stability of lawsonite under high pressure and high temperature conditions.