Implicit-explicit Runge-Kutta-Rosenbrock methods are proposed to solve nonlinear stiff ordinary differential equations by combining linearly implicit Rosenbrock methods with ex-plicit Runge-Kutta methods.First,the general order conditions up to order 3 are obtained.Then,for the nonlinear stiff initial-value problems satisfying the one-sided Lipschitz condi-tion and a class of singularly perturbed initial-value problems,the corresponding errors of the implicit-explicit methods are analysed.At last,some numerical examples are given to verify the validity of the obtained theoretical results and the effectiveness of the methods.