This paper describes the nnmerical analysis of ceiling jets relevant to the "T" pattern flame which accelerates the fire growth due to a flame burning upward along a corner, using a three-dimensional field model. It vas found in calculations that ceiling jets split into two directions along ceiling-wall intersections when a heat source is located at a marked distance from a corner. Isotherms, velocity vectors and isobaric contours of the split ceiling jet showed the configulation similar to the "T" pattern in the experiments of Williamson et al. The "T"-shaped flow was investigated using "open" and "closed" type fire rooms. Further , it was found that the distance between the corner and heat source plays an important role in forming the "T" pattern and that the temperatures and pressures at the ceiling corner oscillate regularly.