Based on measurement of peak transmittance of narrow band filter, the extinction coefficient of a weakly absorbing thin film can directly be calculated. This is a new method used for determination of the extinction coefficients of weakly absorbing thin films with ease to do and very high precision. The principle of measurement is illustrated and the precision of this method is analyzed. The effect of preparation parameters on the extinction coefficients of TiO2 films deposited by ion-beam-sputtering is discussed. The experiment shows that the deposition rate and ratio of Ar∶O2 during the deposition are important factors. When the rate reduces from 0.35 nm/s to 0.23 nm/s, the extinction coefficient of TiO2 film falls from 3.9×10-5 to 2.1×10-5; and when the ratio of Ar∶O changes from 1∶2 to 1∶4, the extinction coefficient of TiO2 film falls from 5.6×10-5 to 2.3×10-5. In addition, the optical loss increases with substrate temperature rising. It is proved that the determination of the extinction coefficient of weakly absorbing thin films, by means of measured peak transmittance of narrow band filter, is a successful method.