We prepared blended oil using soybean oil (SR10) and rapeseed oil (SR01), and evaluated the quality of deteriorated pure and blended oils (with weight/volume ratio of soybean oil to rapeseed oil of 2:1, 1:1, and 1:2) by heat treatment and aeration. The ratio of the acid value following deterioration (AV) to its initial value (AV₀) i.e., the value of AV/AV₀ was in the order SR10 (3.29) << SR01 (7.50). On the other hand, the ratio of the carbonyl value after deterioration (CV) to its initial value (CV₀) i.e., the value of CV/CV₀ decreased with increase in the proportion of rapeseed oil in the blended oil (SR01 (3.12) << SR10 (4.15)). We thus establish that the fatty acid component of the blended oil is a very important factor in the deterioration of edible oil by heat treatment and aeration. Next, we evaluated the improvement in the quality of deteriorated oil by calcium silicate adsorbent (Ca:Si = 1:3, CAS30) treatment i.e., adsorption. It is established that CAS30 exhibited the ability to remove AV and CV from deteriorated oil (treatment temperature: 80 °C < 100 °C). Moreover, we investigated the removal mechanism of AV and CV, and determined good positively or negatively linear relationship between AV or CV removal by CAS30 and the proportion of unsaturated fatty acid in deteriorated pure and blended oils. In summary, the results obtained from treatment with CAS30 provided useful information on improvement in the quality of deteriorated blended oil.