The standard enthalpy of formation of substance in its standard state is zero.
Enthalpy is defined as a thermodynamic quantity that describes the energy of a system. For substances in their standard state, the enthalpy of formation is zero.
The standard state of a substance is defined as the state in which it is found under standard conditions. The following substances has their standard ethalpy of formation as zero or not zero;
Zero enthalpy of formation Non zero enthalpy of formation