With the high density semiconductor technology wireless sensor nodes have several sensors or functions, and are operated by the complex control programs. These sensor nodes operated by low power, need to be reprogramming to change their functions or add additional functions because of environmental changes or control program bugs until their batteries last. This paper presents a firmware upgrade model for wireless sensor nodes remotely and verifies its results by simulations, The simulation results show that upgrade performances are changed by the density of sensor nodes and by selection methods to retransmit the upgrade data to other nodes, Under no bit errors (BER), the upgrade performances are increased as the packet sizes are increased. But there is no performance improvement, when the packet sizes are 160Bytes or more, When BER is higher than $3.0{ imes}10^{-4}$, the upgrade performances are decreased as packet sizes are increased.