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The defects and electron densities in Ti50Al50, Ti50Al48Mn2 and Ti50Al48Cu2 alloys have been studied by positron lifetime measurements. The results show that the free electron density in the bulk of binary TiAl alloy is lower than that of pure Ti or Al metal. The open volume of defects on the grain boundaries of binary TiAl alloy is larger than that of a monovacancy of Al metal. The additions of Mn and Cu into Ti-rich TiAl alloy will increase the free electron densities in the bulk and the grain boundary simultaneously, since one Mn atom or Cu atom which occupies the Al atom site provides more free electrons participating metallic bonds than those provided by an Al atom. It is also found the free electron density in the grain boundary of Ti50Al48Cu2 is higher than that of Ti50Al48Mn2 alloy, while the free electron density in the bulk of Ti50Al48Cu2 is lower than that of Ti50Al48Mn2 alloy. The behaviors of Mn and Cu atoms in TiAl alloy have been discussed.