Using X-ray diffraction combined with Rietveld method and chemical analysis with hydrofluoric acid (HF) solution, the content and composition of glass phase of a class F fly ash are quantified, and the initial Si/Al ratio of glass phase is calculated as well. The dissolution of Si4+ and Al3+ in different chemical solution (HF, NaOH, and NaF) in various concentrations and with different reaction time of NaOH solution was studied. The dissolved Si/Al ratio was compared with the initial value, and following conclusions are drawn: (1) only a very small part of Si and Al dissolved in NaOH solution, knowing as available Si and Al; (2) the dissolution is influenced by alkali concentration. When the concentration of NaOH was 10M, the dissolved Si/Al ratio at 5, 15, and 30 min was lower than initial value, but increased greatly at 240 min, which means Al is more prone to dissolve than Si, due to the weaker Al-O bonds than Si-O bonds. However, when the alkali concentration was 15M, the dissolved Si/Al ratio increased, and the dissolution of Si and Al both get accelerated.