Titanium and its alloys have been widely used in automotive industry and aerospace field due to their high mechanical strength and low density. It has been known that a-Ti has an hcp crystal structure and silp in hcp structure is limited because of only 3 independent slip systems. Therefore, twinning is active in hcp structure and the deformation behavior of hcp metals is very complex by the presence of both dislocation slip and twinning. In sub-micron sized a-Ti sample, deformation twins are difficult to produce and the deformation mechanism is mainly dislocation slip. However, it is hard to identify the activated dislocation slip system in a-Ti, as a few avaliable slip planes is corresponding to one slip direction. Usually there are two ways to identify the activated slip systems. One is to deduce the slip plane and the slip direction based on the loading direction and the crystal orientation. But this method is not accurate because of many possible groups of slip planes and slip directions in hcp structure. The other one is judging the Burgers vector of the dislocation under certain diffraction vectors based on Bragg's law by using TEM. It takes time and can only determine the slip direction of dislocation. Therefore, it is important to find an effective method to identify the active slip system more simply and accurately during deformation process. In this work, a nanometer sized tensile sample of a-Ti single crystal was fabricated by using focused ion beam (FIB) technique. In situ tensile test was carried out along [2110] of a-Ti sample by using a homemade bimetal stretching device in TEM. It has been found that three types of the dislocations, one prismatic dislocation and two pyramidal dislocations, were activated in order with strain increasing during tensile process.The Burgers vectors of dislocations were determined by two-beam diffraction contrast imaging in TEM. For hcp structure, one Burgers vector may have the characteristics of a variety of slip planes. By EBSD technique, the crystalline orientation and the loading direction in TEM were indexed accurately and Schmid factors for all the possible slip systems were calculated corresponding to each Burgers vector. Then, the activated slip systems during in situ TEM tensile process are determined by Burgers vector and Schmid factor. This work offers an effective method to identify the activated slip system during tensile process and get more understanding about the plastic deformation mechanism of a-Ti and hcp metals.