The Raman spectra and in-situ X-ray diffraction patterns of synthetic merrillite, Ca9NaMg(PO4)(7), have been measured up to 1173 K at ambient pressure. No phase transformation was observed in this study. The effect of temperature on the Raman spectra shows that all observed active bands continuously shift to lower wavenumber with increasing temperature. The temperature dependences of v(3) anti-symmetric PO stretching modes are larger than those of v(1) symmetric P-O stretching, v(4) and v(2) O-P-O bending modes and the external modes. Lattice parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 4.86(5) x 10(-5) K-1, 1.46(1) x 10(-5) K-1 and 1.94(5) x 10(-5) K-1 for V, a and c, respectively. Thermal expansion of merrillite shows an axial anisotropy with a smaller expansion coefficient along the alpha-axis. The average isobaric mode Gruneisen parameter for the internal PO4 modes of merrillite has been calculated as 0.57.