Polydimethylsiloxane (PDMS) and graphene-PDMS nanocomposites (GP) have been widely studied because of their excellent properties, of which the elastic modulus is very important for various applications. Here, the dependence of the elastic modulus of properly cured PDMS and GP on the temperature has been investigated. For both PDMS and GP, a critical temperature (T_c) has been found, which originates from the strong affinity of PDMS chains to the PDMS network and graphene sheet, as suggested by molecular dynamics simulation. Graphene inhibits the cross-linking of PDMS close to its surface, which leads to the reduced elastic modulus of GP (E_GP). Only when the temperature is aboveT_c,E_GPincreases with temperature. This is the result of the entropy elasticity of PDMS and the re-initiated cross-linking of PDMS. However, the elastic moduli of PDMS and GP are independent of the temperature belowT_c. Here, the study provides a guideline for the preparation and using of PDMS and its composite at various temperatures.