Abstract: The effect of the preweld cleaning and heat treatment of titanium plate as well as the duration of the weld puddle on the pore formation in titanium weld has been investigated in this paper. The duration of the weld puddle is controlled by either changing the weld travel speed or varying the duration of the argon arc spot welding. Both the fracture surface with the pores and the internal wall of the pore chamber were observed by using scanning electron microscope. We discovered the micro-pores with about 1 micron in diameter and individual solid particle on the wall of most of the pores. Analyzing the process of fhe pore formation and the works which have been published, we further develop the view point of pore nucleation with regard to the pore formation. We consider that both the above mentioned micropores and particle act as pore nuclei, however, the former plays a more important role. The difference between the pore formation near the fusion boundaries and that in the middle portion of the weld has been discussed. The reduction of the number of pores in the weld by preweld cleaning and heat treating titanium plate is due to reduction of the number of pore nuclei. The view point of pore nucleation can satisfactorily explain the phenomena which have not yet been solved.