The Thermax 700 thermo gravimetric analysis (TGA) instrument is introduced for the investigation of the weight change of SAPO-34 catalyst during methanol-to-olefin (MTO) process. By the use of a special sample basket with screen bottom, the TGA instrument can be viewed as fixed-bed reactor, while the weight change of SAPO-34 during MTO reaction can be recorded online. It is noticed that the SAPO-34 catalyst experiences a procedure of dehydration as furnace temperature rises; hence the weight increase of SAPO-34 during MTO reaction should be calculated based on the weight after dehydration. It is checked that the increase of weight is mainly coke ignore of adsorbates. In the range of 648∼773K temperature and 7.08∼35.9h-1 (WHSV) space velocity, the maximum coke deposition is roughly 25% independent of reaction condition. The coke formation experiences a stage of fast increase soon after reaction initiates, while slows down apparently after 18% coke is deposited on the catalyst. Higher temperature, space velocity and methanol partial pressure accelerate the coking rate. Temperature plays an important role in the type of coke formation, which can be distinguished directly by the colour of completely deactivated catalyst. The deactivated catalyst in lower temperature experiences a weight loss with partly recovery of catalyst activity as temperature rises in the hydrothermal atmosphere. The phenomenon of weight loss with catalyst regeneration will not occur as reaction temperature decreases for deactivated catalyst, as well as the different DTA curves for coke burning, are also evidences for the different type of coke formation on SAPO-34 catalyst in different reaction temperature.