The objective of this research was to find an appropriate mixing ratio of concrete with oil palm to produce concrete masonry units with properties equivalent or better than other conventional masonry materials. By trying out method the best proportion of Portland cement: sand: oil palm ash was found at 1:1:2 by weight. The ratio yielded a lightweight concrete with an average density of 766 kg/m3 and average compressive strength of approximately 90 kg/ cm2 which is higher than the minimum requirement of Thai Industrial Standards (TIS) 58- 2533 (1990) of 75 kg/cm2. Water absorption of this proportion was at an acceptable standard level of 20% by weight. From cost analysis the cost of a wall made with the obtained masonry units was about 100 baht per square meter cheaper than lightweight masonry units sold in the market. However the heat conductivity (K) was similar (0.194 W/mk). Moreover the manufacturing process of these masonry units is simple and can be run as a small local enterprise.