Comments are given on the various physical definitions of the force constant of propellant. As there are more or less defects in these definitions, two new definitions are presented to reflect the physical essence of propellant force more perfectly and could well compatible with the mathematical expression. Comments are given on the various physical definitions of the force constant of propellant. Two new definitions are presented to reflect the physical essence of propellant force more perfectly and could well be compatible with the mathematical expression.