Seven stage cascade impactor was used to collect samples of particulate in the spring and fall 1996 in Beijing in order to study the alkaline component of aerosol in different size range, The capacity of calcium salt and magnesium salt acting as a buffer and neutralizing the acid was compared and included that the calcium salt is the main alkaline component. The acid-soluble Ca2+ ions concentration has strong positive regression with the quantitative change in H+ ions concentration after the particulate was dissolved by acid, and the ratio of alkaline calcium salt to acid-soluble calcium salt didn't show seasonal variations, from coarse mode size range to fine mode size range (from first to seventh stage), this ratio are correspondingly 0.84, 0.76, 0.71, 0.59, 0.53, 0.57, 0.28 and value increase with the size growth. The size distribution of alkaline calcium salt and acid-soluble calcium salt in two seasons are almost same. Based on the experimental result, the content of alkaline calcium salt in particulate matter in different size range in spring and fall at Beijing are calculated.