To prepare high strength SiC fibres that cannot be broken with a shear force because of the absence of bend portions, a firing tension is necessary in the pyrolysis of polycarbosilane bundles. However, up to now, no theoretical discussions were released on the tension schedule. Some theoretical aspects, therefore, such as constitutive model, Weibull parameters, peak load/stress, survival probability and upper limit tension were discussed and a theoretical tension schedule was proposed in the paper.