In the low carbon economy， it is imperative to reduce the carbon dioxide（CO2） emission in the power sector. And the generation expansion planning（GEP） in low carbon is one of the key links to achieve CO2emission reduction.Cap and Trade is recognized as an effective measure to reduce CO2 emission and gain low carbon benefits. The carbon allocation mechanism is the core. The impact of low carbon factors on GEP is analyzed from three aspects， i.e. policy， technology and market. A carbon allocation mechanism based on area comparison is proposed and the low carbon generation expansion planning（LCGEP） model is developed by introducing low carbon factors into the traditional GEP model. Finally， a modified particle swarm optimization algorithm based on differential evolution（DEPSO） is employed to solve the planning problem in an example which is used to prove the proposed LCGEP model. The simulation result of the example shows the correctness and applicability of the model， providing current GEP work with beneficial reference.