Using synthesized MgCO3 and reagent-grade MnCO3 as starting materials, a series of Mg1-xMnxCO3 carbonate solid solutions were synthesized by a simple solid reaction under high-temperature-pressure conditions of 3 GPa and 800°C for 4 h. The phase compositions of as-synthesized Mg1-xMnxCO3 samples were investigated by powder X-ray diffraction (XRD); no impurities were observed. The lattice parameters were refined and showed a linear relationship as a function of the Mn2+ content, which is expected to be in accordance with the ideal solution model. Based on this, high- temperature XRD measurements were carried out to further study the thermal expansivity of Mg1-xMnxCO3. The axis thermal expansion coefficients (αa and αc) and the volumetric thermal expansion coefficient αV for Mg1-xMnxCO3 were quantified as αa =7.41×10-6/°C, αc=2.37×10-5/°C and αV=3.86×10-5/°C for x=0.0; αa =6.67×10-6/°C, αc=2.31×10-5/°C and αV=3.67×10-5/°C for x=0.1; αa=6.61×10-6/°C, αc=2.35×10-5/°C and αV=3.59×10-5/°C for x=0.3; αa=5.91×10-6/°C, αc=2.40×10-5/°C and αV=3.58×10-5/°C for x=0.5; αa=5.47×10-6/°C, αc=2.53×10-5/°C and αV=3.61×10-5/°C for x=0.7; αa=4.76×10-6/°C, αc=2.55×10-5/°C and αV=3.52×10-5/°C for x=0.9; αa=4.18×10-6/°C, αc=2.50×10-5/°C and αV=3.35×10-5/°C for x=0.3. The thermal expansion coefficients (αa, αc and αV) can be fitted with a symmetric cubic function of the Mn2+ content as αa=7.34×10-6 -7.06×10-6x+1.21×10-5x2-8.19×10-6x3; αc=2.37×10-6-7.94×10-6x+2.57×10-5x2-1.64×105x3; αV=3.85×10-5-2.08×10-5x+4.59×10-5x2-3.01×10-5x3.