Abstract: In allusion to one micro wireless sensor, an electrically small loop antenna working at ISM 433.05 ~434.79 MHz frequency band is designed by loading lumped elements, structure optimization and encapsulated with the radio frequency circuit. The size of antenna is only 18 mm′1 mm′13 mm(0.02l′0.001l′0.002l, l:the wavelength). By the means of adjusting the values of capacitance and inductance, good simulated impedance can be achieved, which is beyond 7 MHz of -10 dB impedance bandwidth and gain of antenna is over 0.5 dBi. Then a prototype of antenna has been fabricated and tested. The measured results are according with the simulated, where the measured bandwidth is 4 MHz and the radiation efficiency is beyond 20%. The radiation pattern is almost omnidirectional. Compared to the traditional antennas working at 434 MHz,it has advantage of smaller size and compact, easy to tune and has good prospects for the wearable devices, the micro sensor and intelligent hardware.