This paper presents the experimental results of the investigation conducted on unfired building bricks (UBB) made from thermal power plant ashes. Four brick mixtures with a water-to-binder (w/b) ratio of 0.40 and fly ash content of 30, 50, 70, and 85% amount of total binder was designed to evaluate the effect of fly ash content on properties of UBB. Another mixture was designed with a w/b ratio of 0.35 to investigate the effect of the w/b ratio on properties of UBB. The obtained results showed that as an increment in fly ash content, the compressive strength and unit weight of bricks decreased, while water absorption and water permeability of bricks increased. Adverse results were obtained with a reduction in the w/b ratio. Under the scanning electron microscope (SEM) investigation, many free-fly ash particles were observed in brick samples containing a high level of fly ash, resulting in poor properties of these bricks. However, if the use of a suitable fly ash content combined with a proper w/b ratio, the quality of thermal power plant ashes bricks significantly increased, and they can be applied in real practice.