In this paper, we consider a free boundary tumor model with a periodic supply of external nutrients, so that the nutrient concentration satisfies = (t) on the boundary, where (t) is a positive periodic function with period T. A parameter in the model is proportional to the "aggressiveness" of the tumor.If 0 <̃< min 0≤t≤T (t), wherẽis a threshold concentration for proliferation, Bai and Xu [Pac J Appl Math. 2013;5;217-223] proved that there exists a unique radially symmetric T-periodic positive solution ( * (r, t), p * (r, t), R * (t)), which is stable for any > 0 with respect to all radially symmetric perturbations. 17 We prove that under nonradially symmetric perturbations, there exists a number * such that if 0 < < * , then the T-periodic solution is linearly stable, whereas if > * , then the T-periodic solution is linearly unstable.