The crystal structure of a Tl-Ca-Ba-Cu-O compound in Tc onset=155 K and Tc zero=123 K superconducting specimens was characterized by transmission electron microscopy. Convergent beam electron diffraction analysis established that the phase is of P4/mcc space group. The lattice parameters of the phase were determined to be a=b=0.394 nm and c=3.95 nm. The phase was found to be primarily of a ten-subcell structure by both high-resolution lattice imaging and diffraction pattern analysis. The composition of the grains with the ten-subcell structure being predominant was measured by energy dispersive spectrometry to be TlCa2Ba 3Cu4Ox. Polytypes of the phase with four- and five-subcell structures were also observed. The five-subcell structure was identified to be of P4/mmm space group with a=b=0.394 nm and c=1.97 nm. The compositions of the specimens were measured to vary in composition ratios of Tl, Ca, Ba, and Cu from 1:2:2:3 to 1:2:3:4. The results strongly suggested that the TlCa2Ba3Cu4Ox phase is a high Tc superconducting phase with an exciting possibility that superconducting transition occurs at 155 K for this phase.