Low dielectric constant materials (low-k) as interlayer dielectrics in back-end-of-line (BEOL) interconnects of integrated circuits can provide lower parasitic capacitance between the metal lines. However, the interaction between copper (Cu) conductors and low-k dielectrics is a critical issue. In this study, effects of Cu diffusion on the electrical and reliability characteristics of low-k dielectrics were investigated using Cu/low-k/Si capacitors. Low-k dielectrics with the porosities ranging from 0 to 35 % were used. The thermal annealing with different temperatures and times was applied to drive Cu diffusion. The experimental result showed that the Cu diffusion depth increased with the annealing temperature and annealing time in all low-k films and is dominated by the annealing temperature. Moreover, in the low-k film with a higher porosity, the Cu diffusion rate is faster. The Cu diffusion in the low-k film induced an increase in the leakage current density and a degradation in the dielectric breakdown field and exhibited a positive correlation. Furthermore, it was found that the dielectric breakdown failure time are proportional to the Cu diffusion depth for all low-k films. Therefore, this study points out an important conclusion that a fast reliability evaluation on the Cu/low-k interconnects can be done by measuring the Cu diffusion depth.