The use of unfired building bricks (UBB) to replace conventional fired clay bricks is an effective way to reduce the negative effects on the environment. Moreover, utilization of fly ash (FA) to partially replace cement in UBB significantly reduces the amount of CO₂ emission to the atmosphere. This study investigates the possible application of raw FA from Nghi Son coal power plant in the production of UBB. The FA was used to replace 0%, 15%, 30%, and 50% cement in the brick mixtures. The effect of FA content on engineering properties of the UBB was evaluated. Analysis of cost and the optimal mixture was also conducted. Test results indicate that all of the brick samples had technical properties satisfying the requirements of TCVN 6477-2011. Moreover, this study found that increasing the amount of FA resulted in reducing compressive strength, bulk density, and cost, however, increasing the water absorption of brick.