This paper investigates the characteristics of both efficiency and voltage regulation of a practical single-phase distribution transformer when the shape and dimension of the laminated core are varied. Typically, the iron core of a traditional transformer is composed of E- and I-shape laminations to reduce core loss. The reluctance for the magnetic-flux path inside the iron core of a transformer is significantly affected by the stack, shape, and magnetic property of the laminated core as well as the effective length and area of the magnetic circuit. This paper simulates the magnetic-field distributions of a practical single-phase distribution transformer using a simulation software of FLUX 2D based on finite-element analysis (FEA). Both efficiency and voltage regulation of six different improved E- and I-shape laminated cores are evaluated and compared for future design of single-phase distribution transformers with higher efficiency. The measured inrush currents of the studied distribution transformer subject to various transient conditions are also compared with the simulated ones to validate the feasibility of the simulated model.