A typical QSO spectrum usually shows blue color. Recently, a group of QSOs is discovered with red color. These QSOs are considered to be reddened by dust obscuration. Most previous studies on red QSOs rely on photometric selection, which implies that photometric data from the same filter representing different waveband properties at different redshifts. In our study, we developed a new method for classifying red QSOs with a statistical definition based on "relative spectral flux" at rest frame. The advantage is that we could find "slightly-red QSOs", which can not be found through photometric selection. We investigate the properties of our red QSO samples using SDSS, VLA, and WISE data at z=0.3-1.2. We found that our red QSO samples at lower z and at higher z show different properties. There might be at least two types of red QSOs in our red samples. One type might be associated with dust obscuration, but the other type might not be.