This study re-examines the roles of the gravitational constant G, the vacuum permittivity ε₀,and the vacuum permeability μ₀ while preserving the historically established forms ofphysical laws, and attempts a redefinition of two fundamental SI units: the Coulomb andthe kilogram. As a result, both the Coulomb and the kilogram are shown to be expressiblesolely in terms of length (meter) and time (second). Furthermore, transforming to the Space–Time Unit System yields a fundamental relationbetween the electron mass m₀ and the electron charge e₀, m₀ = e₀ c from which a new dimensionless constant β inevitably emerges, linking classical mechanicswith electromagnetism. This work positions β as a new universal constant, standing alongsidethe fine-structure constant α. In the corresponding natural-unit formulation, the classical electron radius arises naturallyas the fundamental length scale. By adopting this length as the reference unit, the speed oflight, electron charge, electron mass, and electron rest energy all collapse to unity, leavingonly the Planck constant ℏ as the reciprocal of the fine-structure constant α. The Space–Time Unit System presented here reconsiders the structure of physical units fromfirst principles, revealing a new universality that spans classical mechanics,electromagnetism, and quantum mechanics, and extending the conceptual foundation ofnatural unit systems.