Purpose: As governments force electricity producers to use more renewable energy sources, over a hundred thermal power plants in high-income countries turned to biomass as a partial or complete replacement for coal. Is the co-firing technology appropriate for Vietnam? Method: The technology assessment study is conducted by building an integrated lifecycle model of the sector, tracking material and financial flows from fuel sourcing to airborne emissions, simulating the economics, environmental and social implications of blending 5% of rice straw in two different existing coal power plants in Vietnam. Findings: The business value of co-firing is positive –straw is cheaper than coal–. It is likely not large enough to motivate the stakeholders. Co-firing creates an external social benefit by reducing air-borne pollution and creating jobs. It reduces the pollution caused by open field straw burning. We found the external social benefit to be several times larger than the private business value. Within that external benefit, the social value of avoided SO2, PM2.5 and NOx emissions dominates the social value of avoided CO2 emissions. The net job creation effect is positive: collecting straw creates more employment than using less coal destroys. Originality and limitations: This is the first technology assessment of co-firing biomass in coal power plants in Vietnam and one of the first for a subtropical middle-income country. The study only considers rice straw, and it does not address the role of government nor the biomass market functioning. Conclusion: The price of coal is the primary determinant of co-firing business value. There is an empirical economic justification for a public intervention to promote co-firing biomass in Vietnam. Local air quality goals, rather than greenhouse gas reduction policy, can justify such regulations.