Floorplanning is an indispensable step in very large scale integration (VLSI) design flow. Fixed-outline floorplanning is a challenging problem because it requires additional constraint that the floorplanning must meet both width and height criteria. In this paper, a modified simulated annealing (MSA) algorithm is presented with a new cost function for this problem. In our algorithm, a two-step strategy is used to improve the efficiency of the algorithm and balance the main function and secondary function. In addition, a local search strategy is employed to make sure a feasible solution can be found. Experimental results indicate that the success rate of each benchmark is 100% in deferent aspect ratio