The dust grains play important roles in the interstellar processes, but the identification of grain material is a long standing problem in the present-day astrophysics. The AGB stars are one of the major sources of dust grains and detailed studies of their IR spectra are of particular importance for the identification of the material. The IRAS LRS data base, in fact, has provided very valuable information on the circumstellar dust grains. It has shown that the IR spectra of AGB stars have many varieties with several unidentified features (UFs) and that several kinds of dust grains seem to be forming. We propose to study systematically with SWS the UFs commonly observed in two different groups of AGB stars. In each group the characteristic feature is observed in many stars, which may indicate common properties of circumstellar dust grains: For carbon-rich stars, we focus on the 8-9 micron feature; for oxygen-rich AGB stars, we concentrate on narrow features superposed on the 10 micron silicate emission. The aim of our proposal is twofold: 1) to obtain the detailed profile of these UFs in selected wavelength ranges; 2) to explore the entire spectrum between 2.4 and 45 micron and to search for any other features which may be associated with these UFs. With this study we will be able to identify the circumstellar dust grains and we expect to better understand their properties.