The embrittlement by phosphorus and sulfur and its prevention by lanthanum and calcium have been investigated in 93W-4.9Ni-2.1Fe heavy alloy. Powder compacts have been sintered at 1475℃ for 60 or 100 min in hydrogen, heat treated at 1150℃ for one hour in nitrogen, and then water quenched. Toughness decreases drastically with phosphorous or sulfur addition of a few tens ppm, in contrast to much less sensitive decrease of tensile properties. Auger electron spectroscopy and scanning electron micrography have revealed that the phosphorous and sulfur segregate at W/matrix and W/W interfaces and cause brittle fracture. Addition of lanthanum to specimens containing 150 ppm, phosphorous suppresses the segregation of phosphorous at the interfaces by forming La-O-P base compounds and thus prevents the brittle failure. The impact energy of un-doped heavy alloys which did not show any P embrittlement after water quenching decreases drastically with decreasing the cooling rate from the heat-treatment temperatures or aging at 600℃ for 1h. The serious reduction of impact energy can be avoided by adding La or ca which forms P and O containing compounds. Calcium and lanthanum is also observed to suppress the sulfur segregation in S-doped alloys by forming S containing Ca or La compounds, and therefore prevent the sulfur embrittlement. The addition of lanthanum and calciun have ensured good mechinical properties of heavy alloys.