A sustained-release tablet formulation should ideally have a proper release profile insensitive to moderate changes in tablet hardness that is usually encountered in manufacturing. In this study, matrix aspirin (acetylsalicylic acid) tablets with ethylcellulose (EC), Eudragit RS100 (RS), and Eudragit S100 (S) were prepared by direct compression. The release behaviors were then studied in two counterpart series of tablets with hardness difference of three Kp units, and compared by non-linear regression analysis. The release pattern for both the S-containing and RS-containing formulations fitted best in Higuchi model, and the proper equations were suggested. In the EC-containing formulation, Higuchi and also zero-order models were probable models for the release, and a combination equation for the release was suggested. In the S-containing formulation, the release profile was completely sensitive to the hardness change. In RS-containing series, the slope of the release graph did not change due to the hardness decrease, but the y-intercept or the lag time in release was decreased. In EC-containing matrix tablets, both the slopes and the y-intercepts did not change by the decrease in hardness. In conclusion, EC with an amount as little as 10 percent in formulation could make sustained-release aspirin tablets in which the release profile is not sensitive to moderate changes in hardness.