In AlGaN/GaN HEMTs, slow current transients are often observed even if the gate voltage or the drain voltage is changed abruptly. This is called gate lag or drain lag, and problematic in circuit applications. The slow transients mean that dc I-V curves and RF I-V curves become quite different, resulting in lower RF power available than that expected from the dc operation. This is called current collapse, which is characterized by the current reduction in pulsed I-V curves from an off point. Experimentally, it is suggested that the current collapse is more significant when the gate length is shorter. However, few theoretical works have not been made on this point. Therefore, in this work, we have simulated the lags and current collapse in AlGaN/GaN HEMTs, and studied how they are influenced by the gate length. As a result, it has been found that the gate lag increases as the gate length becomes short, but the drain lag is not so dependent on the gate length. Then, the current collapse, which is a combined effect of gate lag and drain lag, tends to increase as the gate length becomes short due to the gate-lag effect.