The electricity generation expansion planning in Thailand has considered greenhouse gas (GHG) emissions directly from the generation stage. The lower-emission power plant, i.e. natural gas, has been chosen rather than an emission-free plant. However, the choice of natural gas has been proven as a high-emission source in terms of full energy chain analysis. The combined process chain and input-output analysis is selected as a tool for comparative assessment of net GHG emissions in the study. The complexity, of energy flows and related emissions, within the economic sectors is facilitated by advantages of the input-output (I-O) model. In the I-O model, the net GHGs emission factors, directly and indirectly, are estimated by using the 1995 Thailand 180x180 input-output table, the national sectoral energy consumption statistic, and the emissions factors recommended by the Intergovernmental Panel on Climate Change (IPCC). Since the sector energy consumption is available at a higher aggregation level of economic sectors than those required by the I-O model, disaggregation of sectoral energy consumption is assumed in the study. Finally, the comparative assessment of net GHG emissions is made between the run-off-river hydroelectricity project and an independent gas-combined-cycle power plant in Thailand. Results are presented of energy intensities and net GHGs emissions from these plants.