Superconductors provide a Hrlde spectrum of posstbdltles for sensing &fferent physical effects and actual values or changes of physlcal quantltles, because their fundamental propeties are strongly influenced by external condltlons The development of h&-T, ceramic superconductors III the form of bulk and fihn samples has broadened the apphcabdlty of these mater& for various kmds of sensors and opened a way to design a new class of detectors Based on some early measurements showmg the strong dependence of the mtcrowave propertles of high-TC compounds on an external magnetic field, our first device apphcation has been deslgned to control a thick-film microwave ring resonator by low magnetic field H In this paper a new and sensltnre magnetic-field detector IS presented, which 1s developed from the prewous work mentioned above The quahty factor Q of a high-I', superconductmg rmg resonator IS observed to increase and then decrease wth the mcrease of the field strength H m the low magnetic-field range Q shows a huge change m the field mterval O-3 mT The value of dQ/dH IS 1 dB/18 IT By usmg an HP Network Analyzer of 1 V/dB for thrs mvestlgatlon, the accuracy of the measurement of output voltage guarantees to detect the magnetic field wth a senslbwty of l-10 nT