A sample with a nominal composition of Hg:Tl:Ba:Cu:C = 1:1:4:2:1 was prepared by a high-pressure synthesis technique. A new phase with a space group of I4/mmm and lattice parameters of a = 3.9256 and c = 38.875 was found in this sample. An energy dispersive X-ray spectrum analysis indicated that the composition of the new phase is not far from the nominal one. A transmission electron microscopic observation revealed that in this phase the stacking order is CO-BaO-CuO 2 -BaO(Hg,Tl)O-(Hg,Tl)O-BaO-CuO 2 -BaO along the c-axis, and there is no superstructure in the ab-plane. A crystal structural model was proposed based on the transmission electron microscopic observation. By using this model, the high resolution transmission electron microscopic images as well as powder X-ray diffraction pattern were simulated. It was found that the simulations are in good agreement with the experimental ones.