Optical emission spectroscopy can be used to monitor the composition of Ti50Ni40Cu10 thin films during sputtering. The sputtering pressure can affect the spectrum intensity of Ni during r.f. magnetron sputtering, but has no obvious effect on that of Ti and Cu. This may be due to the ferromagnetic characteristic of Ni. By choosing peaks of Ti: 365.35 nm, Ni: 341.48 nm, Cu: 327.40 nm, we find that the peak intensity ratios of I-Ni,(341.38)/I-Ti,I-365.35 and I-Cu,(327.40)/I-Ti365.35 remain constant in the range of 20-50 mTorr Ar pressure. The intensity ratio of these peaks is found to be proportional to the composition ratio of thin films with the relations: Ni(%)/Ti(%) = 2.3817(I-Ni/I-Ti) - 0.8141 and Cu(%)/Ti(%) = 0.7427(I-Cu/I-Ti) + 0.1106. Hence, the composition of sputtered thin films can be predicted by monitoring the intensity of light emission from the sputtering plasma.