This research aims at obtaining rice husk ash and rice straw ash by burning rice husk and rice straw. The agricultural by-products, examining the flowability of fresh concrete. Mechanical properties and the possibility to react to pozzolan of hardened concrete, and the length change rate by dry shrinkage, and assessing whether they are able to be applied as the mixture material of concrete. In case of the slump of fresh concrete, rice husk ash had better liquidity and the possibility to make forms with it than rice straw ash. The amount of air was proper for KS standard at 5% of rice husk ash and rice straw ash. In terms of the compressive strength of hardened concrete, the strength of the RHA of early age was smaller than the standard, and the strength of rice straw ash at the replacement rate of 5% and 10% was bigger than the standard, except for that at the replacement rate of 15%. The strength of RHA of standard age and 10% of RSA were bigger than the standard at all replacement rates. The pozzolan activity factor was higher than the standard at the standard age, regardless of the type of mixture material and replacement rate, so it can be concluded to have the possibility to react to pozzolan. The analysis result of SEM photographing proved that rice straw ash is a heterogeneous material with mixed big and small granules, and their linkage of reaction is long and surface is rough, which is estimated to affect the manifested strength.