It is well known that the strength of welded joint is largely influenced by the cooling cycle around the bond. Theoretical solutions in heat conduction are very useful to estimate the cooling cycle of the welded joint with simple configuration. However, numerical methods must be introduced when dealing with a complicated-shaped welded joint. In this paper, the cooling process of T-shaped welded joint is analyzed by means of 3-dimensional F.D.M. where irregular meshes are used in the plane which is perpendicular to the direction of welding. The governing equation here is for a quasi-static temperature field because heat input by arc can be treated as moving heat source. The effects of plate thickness and height on the cooling cycle are evaluated, and the accuracy of the numerical solutions are examined by the experiment of CO2 arc welding.