With the increasing use of distributed generator（DG）， various uncertainty factors of DG will have more impact on available transmission capability（ATC） assessment. An assessment method of ATC considering DG influences is presented. The uncertainty factors， including installation location， running status， generating power of DG， constitute random variables of the system. The stochastic problems are solved using Monte Carlo simulation， combined with traditional Newton-Raphson power flow method to forecast some possible operation state of the system. Some probability indices for ATC evaluation are defined. Tests in the IEEE 30-bus system show the necessity to consider the influence of DG and the practicability of the presented approach.