Abstract: Vibration frequency modes of quasi-one-dimensional SiO2 clusters with one chain, two chain, three- and four- membered ring structures were calculated according to the density-functional theory. The bending vibration at low frequency (less than 300cm-1) was emphatically investigated in the infrared spectrum with different conformation. This type of vibration mode shows strong size effect dependent on frequency and intensity, in which the frequency has exponential relations with the length of clusters. Furthermore, the variation of the exponent with diameter is corresponding to the geometry dimension of the clusters. It is found that such infrared vibration exists only at the length range of 0.8nm～5.5nm. The results reveal that the criterion can be used to characterize the specific length of nano-cluster in experiment and theory.