This paper proposes a chance constrained formulationto tackle the uncertainties of load and wind turbine generatorin transmission network expansion planning. A combined MonteCarlo simulation/analytical probabilistic power flow analysismethod is first presented to obtain the probability density functionof wind turbine generator output. The paper then shows thedevelopment of the chance constrained formulation with the inclusionof the wind turbine generator probability density functionand probabilistic power flow in the formulation. The proposedformulation is more computationally efficient and can deal withuncertainties in transmission network expansion planning. Thepower of the new method is shown through the application of theformulation to two test systems.