We report an ab initio study on the electronic properties of 5f states in UX3( X=Al,Sn) by full-potential linear muffin tin orbitals L(S)DA calculations. The r elativistic effects which are quite remarkable for heavy atoms such as U, have b een treated by using scalar relativistic and spin-orbital coupling corrections. The calculations presented in this article have addressed following issues:first ly, the numerical results illustrates the different U 5f itineracy in UAl3 and USn3 qualitatively, and then the heavy fermion behavior of USn3 ;secondly, u sing Stuttgart-fatband analysis, we have confirmed the above conclusion quantita tively. In addition to the above results, the calculation involved in this resea rch has resolved the discrepancy between previous density functional theory stud ies on these compounds, especially the band structure dispersion in M-X directio n of simple cubic USn3. In conclusion, this study has approached a mo re precis e description for these uranium compounds on the basis of modern density functio nal theory calculation and described USn3 as a heavy fermion system d ue to its localized U 5f electronic states theoretically.