A prototype of an x-ray microcalorimeter using Titanium/Gold transition edge sensor (TES) for detecting cosmic rays is fabricated and tested. This paper reports first experimental result of the prototype. By using silicon bulk micromachining, freestanding microstructure suspended with fine beams are obtained to achieve thermal isolation from the substrate. A superconductor, Ti in this case, can be used as a very sensitive temperature sensor at the narrow temperature range around its transition temperature. At the low temperatures below 1K, the microstructure with very small heat capacity is expected to be thermally detecting single photons. Design consideration to realize radiation detection with extremely good energy resolution has been taken place. Our tentative goal is to obtain the energy resolution of 20eV for 10keV radiation at 0.5K. We have fabricated a test device of the TES. The sensitivity of it is larger than 1000, which is enough for this purpose. The energy resolution of the prototype of the x-ray microcalorimeter was 550eV for 6keV radiation at approximately 0.5K. This value is smaller than that expected. An optimization of the TES features is still necessary for a good energy resolution.