In this paper, Ni₈₈Cu₁₂ thin films are prepared on Si substrates by radio-frequency magnetron co-sputtering. The effects of film thickness and heat treatment on domain structure and magnetic properties are studied. The Ni₈₈Cu₁₂ films with thickness less than 210 nm show weak in-plane anisotropy. After the vacuum magnetic field heat treatment, the crystal grains of all films grow, and the in-plane anisotropy extents of Ni₈₈Cu₁₂ films with thickness less than 210 nm become weaker. The films have different morphological characteristics at different heat treatment temperatures. The surface of the film is very dense before heat treatment. After heat treatment at 300 °C, it forms island shape due to the solid solution between the grains. Compared with the grains on the surface of the heat-treated film at 300 °C, the grains grow further after being heat-treated at 400 °C. Grown grains cover the gaps between the grains, which makes the film surface flat. The results of in-plane normalized hysteresis loop of the films show that the critical thickness of the stripe domains decreases after heat treatment. The stripe domain structure appears in the 210-nm-thick films without being heat-treated. For the film with the heat treatment, its remanence ratio M_r/M_s decreases, and the saturation field H_s increases, and thus leading the perpendicular anisotropy constant K_p to increase. Therefore, the thickness of the stripe domain in the film after being heat-treated decreases. Compared with the films after being heat-treated, the 105-nm-thick as-prepared Ni₈₈Cu₁₂ film has a maximum natural resonance frequency of 2.1 GHz, which is attributed to the uniaxial anisotropy of the as-prepared Ni₈₈Cu₁₂ film. The saturation magnetization of the 300 °C-treated 105-nm-thick film decreases to 3.01 × 10⁵ A/m. However, the saturation magnetization decreases to 5.9 × 10⁵ A/m after heat treatment at 400 °C. Moreover, the ferromagnetic resonance peak of the film narrows after being treated at 300 °C, and the resonance frequency decreases to 1.95 GHz.