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A comprehensive numerical investigation of the uncoupled chemical, thermal, and transport effects of CO₂ on the temperature of CH₄/O₂ counterflow diffusion flame under high pressure up to 5 atm was conducted. Three pairs of artificial species were introduced to distinguish the chemical effect, thermal effect, and the transport effect of CO₂ on the flame temperature. The numerical results showed that both the chemical effect and the thermal effect of the CO₂ dilution in the oxidizer side can decrease the flame temperature significantly, while the transport effect of CO₂ can only slightly increase the flame temperature and can even be ignored. The reduction value of the temperature caused by the chemical effect of CO₂ grows linearly, while that caused by the thermal effect increases exponentially. The <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>R</mi><msub><mi>P</mi><mrow><mi>c</mi><mi>h</mi><mi>e</mi><mi>m</mi></mrow></msub></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>R</mi><msub><mi>P</mi><mrow><mi>t</mi><mi>h</mi><mi>e</mi><mi>r</mi><mi>m</mi><mi>a</mi><mi>l</mi></mrow></msub></mrow></semantics></math></inline-formula> are defined to explain the temperature reduction percentage due to the chemical effect and the thermal effect of CO₂ in the total temperature reduction caused by CO₂ dilution, respectively. The <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>R</mi><msub><mi>P</mi><mrow><mi>c</mi><mi>h</mi><mi>e</mi><mi>m</mi></mrow></msub></mrow></semantics></math></inline-formula> decreases with the increase of the pressure, the strain rate, and the CO₂ dilution ratio, while the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>R</mi><msub><mi>P</mi><mrow><mi>t</mi><mi>h</mi><mi>e</mi><mi>r</mi><mi>m</mi><mi>a</mi><mi>l</mi></mrow></msub></mrow></semantics></math></inline-formula> behaves in the opposite manner. In the above conditions, the chemical effect plays a dominant role on the flame temperature reduction.