Unit step responses (USRs) of a USR-measuring system recommended by the International Electro-technical Commission (IEC) have been calculated, using the finite-difference time-domain (FDTD) method, for various conditions of grounding and shield ring. The USR measuring system subject to analysis has a resistor divider of height 3.3m and resistance 9kΩ. The conductivity and relative permittivity of the laboratory floor, on which the USR measuring system is placed, are set to a range from σ = 0.001mS/m to ∞ and a range from ε_r = 1 to 10, respectively. It turns out that USR parameters are not much influenced by either σ or ε_r if σ is higher than or equal to 10mS/m, and they are close to the USR parameters for σ = ∞. When σ is lower than about 0.1mS/m, USR parameters are more influenced by σ and ε_r. Variations in the partial response time T_α, the experimental response time T_n, the settling time T_s, and the overshoot β, due to the differences of σ and ε_r, are 8ns, 14ns, 150ns, and 50%, respectively. Even if the width of horizontal ground metal sheet is extended from w_g = 2 to 3m, these variations are not much reduced. The extension of the width of vertical metal sheet from w_v = 1 to 2m does not much influence the USR parameters, either. Both the installing height of shield ring h and its diameter d influence the USR parameters: T_α decreases with decreasing h and/or d. Note that the FDTD-calculated USR for σ = 0.1mS/m and ε_r = 10 agrees reasonably well with the corresponding measured USR: T_α = 19ns, T_n = 6ns, T_s = 310ns, and β = 53% vs. T_α = 16ns, T_n = 10ns, T_s = 280ns, and β = 44%.