This paper reports on the potential use of fly ash (FA) and residual rice husk ash (RHA) in producing unfired building bricks (UBB) with the application of densified mixture design algorithm (DMDA) method. In this study, little amount of cement (10–15%) was added into the mixtures as binder substitution. Whereas, unground rice husk ash (URHA), an agricultural by-product, was used as partial aggregate replacement (10–20%) in the mixtures. The UBB of 220×105×60 mm in size were prepared and the hardened properties of the bricks were tested including compressive strength, flexural strength, water absorption and bulk density according to Vietnamese standard. Forming pressure of 35 MPa was applied to form the solid bricks in the mold. The test results show that all brick specimens achieved very good mechanical properties. The compressive strength, flexural strength and water absorption of brick specimens were respectively in range of 16.1–22.1 MPa, 2.8–3.5 MPa and 9.5–14.8% and the other properties of the bricks were well conformed to related Vietnamese standard. It was definitely proved many potential applications of FA and RHA in the production of UBB.