This invention proposes a wireless bike brake light that can respond to braking when bikes are in motion. The invention employs a MEMS (micro electro mechanical system)-structured accelerometer chip as sensor to detect related signals. A control circuits unit processes the signals, and then sends appropriate brake light to following vehicles based. The features of the invention are that (1) It can provide extra security when serving as a brake light with independence of braking system, (2) It is easily to install and durable, (3) It can be also applied to such sports as skiing, skateboarding, etc. that are not equipped with apparent brake devices, and thus hard to incorporate brake lights.