Unpolarized infrared spectroscopic study was carried out on thin nanocrystalline anatase TiO2 films with grain size of approximate to 20nm. The transverse optical lattice vibration frequencies are the same as that of single-crystal anatase. The longitudinal optical modes can be detected with an oblique unpolariation at incident angles >= 50 degrees. The broad LO absorption peak is a combination of the A(2u) LO and the E-u LO modes' resonant response to the infrared radiation.