Thermax 700 thermo-gravimetric analysis (TGA) instrument with large weight capacity is introduced for the investigation of the coke formation of SAPO-34 catalyst during methanol-to-olefin (MTO) process in this study. By the use of a special sample basket and relatively higher methanol concentration, the TGA instrument can be viewed as fixed-bed reactor, while both coking rate and reaction results for MTO reaction can be recorded in situ. The influences of reaction conditions over coking rate are discussed, while the effect of temperature is emphasized. Both weight change and color interchanging phenomenon at different reaction temperatures indicate that the severity of hydrothermal atmosphere is partly responsible for the different species of coke formation as temperature varies. A simplified coking model with exponential mathematical expression is established and verified. Coking has a very close relationship with MTO results. The different trend of product distribution as coking occurs proves the selective deactivation phenomenon. DME can be viewed as an indicator of the deactivation level in this process. The coking model is valuable to express the catalyst activity change during reaction as well as kinetic modeling for further investigations, while the catalyst coke content during MTO process can be predicted simultaneously. (c) 2010 Elsevier B.V. All rights reserved.