Abstract: A mathematical model of heat transfer and unstable solidification in the mould of continuous caster for blooms was developed and applied to predict the temperature distribution and solidified shell thickness profile in the mould of continuous caster for blooms. In view of the air gap in the mould and heat transfer is thus non-homogeneous, a heat exchange model related to the air gap is introduced. Meanwhile, the equivalent specific heat method is introduced to deal with the latent heat of fusion in the front of solidification phase. The model has been applied to calculate the temperature profile and solidified shell thickness in the mould of caster with the blooms supplied in situ as specimens. The effects of casting speed on temperature profile and solidified shell thickness were studied, which show that the data obtained by the model are in conformity with experimental measurements.