A numerical approach to modelling of hot gas duct in tuboannular combustor is suggested.The modelling scheme is based on five controlling rules:top and bottom curves on central profile

radii of top and bottom arcs of cross sections

and area of cross sections.A three dimensional hot gas duct is regarded as a stacking of finite two dimensional cross sections constrained by the rules.A brief analysis of computed rules is presented as well.As an example the hot gas duct of SPEY engine is computed by using this method.Comparison between the computed cross sections and real ones obtained from SPEY technical drawings shows that the method is effective and practicable.