A stable, reproducible and bulk superconductor of Tl-Ca-Ba-Cu-O (TCBCO) compound with T0 above 120 K and Tonset=155 K has been prepared. Measurements of electrical and magnetic properties are reported. On the basis of the transmission electron microscopy and X-ray diffraction analyses, the predominant phase was determined to be a body-centered tetragonal structure (a=b=0.557 nm, c=3.77 nm) with a space group of I4/mcm. The composition ratios of Tl, Ca, Ba and Cu of the phase were determined by energy dispersive spectrometry to be 1:2:3:4. A probable structure model involving four layers of Cu-O interposed between Tl-O layers is proposed.