This paper is to study the influences of loading speed on the microstructure and hardness of magnesium alloy formed at room temperature. AZ31 magnesium alloy was compressed using a 1 100 kN servo presser under the crank at constant speed mode and unidirectional deceleration mode and the microstructure evolution and hardness change during the two processes were examined. The results showed that small amount of parallel and thicktwins can be observed in large sized grains at the beginning of both deformation processes. With the increase of strain,the density of the twins increases correspondingly and the twins tend to be stretched. When the metal was deformed in constant speed mode, the twin was long and thin and the distribution was non⁃uniform with the irregularity of interlacing. In contrast, when the metal was formed in the unidirectional deceleration mode, the twin was initially wide but was gradually transformed to long and thin, and the twin distributions were relatively more uniform and intersected. The density of twin increased and the overall hardness decreased when the metal was formed in the unidirectional decelerated mode. However, the hardness of the formed metal became very close under two speed modes when the reduction was approaching 25%.%为研究变速变形对镁合金微观组织和硬度的影响，采用1100 kN机械伺服压力机，对AZ31镁合金在曲柄匀速驱动和曲柄单向减速驱动两种模式下进行了室温压缩实验，分析了两种速度模式不同变形量下试样的微观组织与硬度演化规律。研究表明：两种变速模式下，在压缩初始阶段，均有少量平行孪晶产生于粗大晶粒内，且孪晶宽度大；随着变形量增大，孪晶密度增大且呈细长化。在曲柄匀速驱动模式下，孪晶长而细且分布散乱，无规则交错；而曲柄单向减速驱动模式下孪晶由宽大转变为细长，多组平行孪晶相互交叉，均匀分布且孪晶密度增大。不同变形量下，曲柄单向减速驱动模式下的试样硬度均小于曲柄匀速驱动模式，但在接近极限压缩量（25％）时，其硬度值又趋于相同。
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