In this paper,we investigate dynamical mass generation in (2+1)-dimensional quantum electrodynamics at finite temperature.Many studies are carried out within the instantaneous-exchange approximation,which ignores all but the zero-frequency component of the boson propagator and fermion self-energy function.We extend these studies by taking the retardation effects into consideration.In this paper,we get the explicit frequency n and momentum p dependence of the fermion self-energy function and identify the critical temperature for different fermion flavors in the chiral limit.Also,the phase diagram for spontaneous symmetry breaking in the theory is presented in Tc-Nf space.The results show that the chiral condensate is just one-tenth of the scale of previous results,and the chiral symmetry is restored at a smaller critical temperature.