A computer model is developed to simulate the time-temperature-transformation (TTT-)diagram for the proeutectoid ferrite transformation in Fe-C and Fe-C-X alloys, where X is a substitutional alloying element, from the diffusion growth theory which assumes para- and/or local equilibrium of solute atoms at advancing phase interfaces. The incubation time of ferrite allotriomorph nucleus calculated from the nucleation theory is assumed to be the initiation time of transformation. The influence of the enrichment (or depletion) of solute atoms in the untransformed austenite matrix on subsequent transformation is incorporated by using the quasi-stationary approximation to calculate the diffusion field in the matrix. Both the planar growth and the growth of spherical interfaces (toward the center of sphere) are considered. The model reproduces the basic features of proeutectoid ferrite transformation in iron alloys during isothermal holding and can be used to predict approximately and quickly the TTT-diagram for the transformation when the alloy composition and the austenite grain size are given.