We have performed large-scale CIV3 (Configuration Interaction Version 3) calculations of excitation energies from ground states for 141 fine-structure levels as well as of oscillator strengths and radiative decay rates for all electric-dipole-allowed and intercombination transitions among the levels of the (1s²2s²2p⁶) 3ℓ², 3ℓ3ℓ' and 3ℓ4ℓ configurations of Cu XVIII. These states are represented by very extensive configuration–interaction (CI) wavefunctions obtained using the CIV3 computer code of Hibbert. The important relativistic effects are included through the Breit–Pauli approximation. In order to keep our calculated energy splittings as close as possible to the experimental values, we have made small adjustments to the diagonal elements of the Hamiltonian matrices. Since mixing among several fine-structure levels is found to be very strong, it becomes difficult to identify these uniquely. Our excitation energies, including their ordering, are in excellent agreement (better than 1.0%) with the available experimental results. From our calculated radiative decay rates, we have also calculated radiative lifetimes of the fine-structure levels. Our calculated oscillator strengths and radiative decay rates are found to be in good agreement with other theoretical results, while the lifetimes agree very well with the experimental values (wherever available). In this calculation, we also predict new data for several fine-structure levels where no other theoretical and experimental results are available.