In this paper,some proposals to explain the emission properties of the La-WC cathode and itsresistance to poisoning of oil vapour are presented and they are demonstrated to be correct by experiments.In the operating temperature range of the La-WC cathode,lanthanum would react chemically with tungstencarbide and form La_2C_3 which is a kind of good emissive material with a work function approximate to thatof lanthanum and can also combine with lanthanum into a eutectic.The eutectic easily wets the tungstenmatrix and migrates over its surface to form a thin emission film.As a result,cathode emission is greatlyincreased.Morever,the La-WC cathode has the capability to resist poisoning of oil vapour.The reason isthat the carbon from oil vapour reacts with lanthanum in the eutectic,thus forming La_2C_3 which is also agood emissive material. In this paper, some proposals to explain the emission properties of the La-WC cathode and its resistance to poisoning of oil vapour are presented and they are demonstrated to be correct by experiments. In the operating temperature range of the La-WC cathode, lanthanum would react chemically with tungstencarbide and form La_2C_3 which is a kind of good emissive material with a work function approximate to that of lanthanum and can also combine with lanthanum into a eutectic. eutectic easily wets the tungsten matrix and migrates over its surface to form a thin film . As a result, cathode emission is greatly enhanced. Morever, the La-WC cathode has the capability to resist poisoning of oil vapor. The reason is that the carbon from oil vapor reacts with lanthanum in the eutectic, thus forming La_2C_3 which is also agood emissive material.