The core material dependency of the building factor in a three‐phase transformer was clarified by a model transformer experiment. The building factor was found to be related to the B10 value (flux density at H=1000 A/m) of the core materials showing that it is dependent upon the degree of Goss texture. The dc simulation of the flux distribution at the instant when the total flux in center limb is zero during a magnetizing cycle showed that the increase in building factor proportional to the B10 value was caused by the increase in flux penetration into the center limb at that instant. The replacement of only the center limb sheets by those with lower B10 values reduced the building factor, as expected.