The primary aim of this study is to investigate the possibility of using unground rice husk ash (URHA), an agricultural by-product, for producing unfired building bricks (UBB) with applying densified mixture design algorithm (DMDA) method. In this investigation, blended fly ash (FA), rice husk ash (RHA) and little amount of cement (5%) was used as the binder material, whereas URHA was used as partial aggregate replacement (10–30%) in the mixtures. By following Vietnamese standard (TCVN 1451:1998), solid building bricks of 220×105×60 mm in size were prepared for investigating the effect of URHA content on hardened properties of the bricks including compressive strength, flexural strength, water absorption, bulk density and void volume. Forming pressure of 35 MPa was applied to form the bricks in the steel mold. The test results show that compressive strength, flexural strength and water absorption of brick samples were respectively in range of 8.4–13.8 MPa, 1.2–2.3 MPa and 9.8–18.2% while the other properties of the bricks were well conformed to corresponding Vietnamese standards. Furthermore, it was definitely proved many potential applications of URHA in the production of UBB.