The axiomatic design approach has contributed to the advancement of design practice by creating good design evaluation criteria based on design axioms. Complexity measure is important in design evaluation in order to simplify the engineering design. In this paper, we use Boltzmann entropy as the complexity measure and derive mathematical relationships between design complexity and various components in engineering design in the context of the axiomatic design approach. Three components of the design complexity are identified, they are variability, vulnerability and correlation, in which the vulnerability is related to the size of the design problem, the interdependency of design parameters and the sensitivity of functional requirements of the design towards the change in design parameters.