C-axis tilted AlN films have been studied for use in quasi-shear mode devices. Highly tilted columnar structure induces the decrease of film density (void between columnar grains) because of the shadowing effect. This could affect the performance of shear mode resonator. In this study, the relationship between the acoustic properties and the grain structure is investigated in c-axis tilted AlN film. Acoustic properties of the resonators were estimated by the characteristic of conversion loss in composite resonator. In c-axis highly tilted AlN film, 1/4 and 3/4 wavelength resonance due to the decrease of shear wave velocity were observed in a number of samples. This phenomenon was not observed in longitudinal wave mode. Porous film structure was also observed in the highly tilted films by across sectional SEM image. Decrease of shear wave velocity would be caused by this structure because shear wave is more sensitive to the porous structure than longitudinal wave.