A nearly single-phase sample of the recently discovered new magnetic material with a nominal composition of Y:Cu = 1:1.8 is prepared by a high pressure synthesis technique. Energy dispersive X-ray (EDX) analysis confirms that the composition ratio of Y to Cu is quite close to 1:1.8. The crystal structure of this sample was determined by means of X-ray powder diffraction Rietveld refinement. According to the refinement results, this structure has a monoclinic symmetry with a space group of I2/a (or C2/c, No.15) and the lattice parameters a = 5.690(5) Angstrom, b = 9.580(9) Angstrom, c = 5.609(5) Angstrom, and beta = 92.470(6)degrees. It is found that about 10% Cu sites are vacancies in this phase. Electron diffraction analysis indicates that the Cu vacancies do not form any ordering arrangement.