Objectives: Amyotrophic lateral sclerosis is the commonest motor neuron disease with incidence of 0.8 person-years in Chinese. Respiratory muscle function has been proposed to be a strong predictor of quality of life (QoL) and survival in ALS. Some studies suggest that most patients with ALS developed hypoventilation when their vital capacity (FVC) is less than 50% of predicted value. However, the incidence of hypoventilation and factors associated with hypoventilation in ALS patients is not clear. Also, there is still no consensus as to which physiologic marker should be used as a trigger for the initiation of non-invasive ventilation (NIV) in ALS patients. The conflicts of studies come from variable subgroup of ALS, pulmonary function at enrollment, techniques used to diagnose ALS, time to apply NIV, and target endpoint. Therefore, this project aimed to study ALS patients who had relatively preserved respiratory muscle function and no respiratory failure at clinical to achieve three goals: (1) To determine the incidence of hypoventilation in ALS patients (2) To identify the clinical characteristics and risk factors associated with hypoventilation in ALS patients (3) To determine the effect of early intervention with NIV on the prognosis of ALS patients Study design: Randomized, controlled trial Participants: ALS patients whose FVC 40%-80% of predict, Pimax <60mmHg, and daytime PaCO2<50mmHg Protocol: Eligible patients with whole-night polysomnography (PSG) and transcutaneous CO2 (PtcCO2). Enrolled patients were randomized to standard treatment or NIV. The primary endpoint of prognosis was survival. The secondary endpoint was changes of PtcCO2 and PaCO2, unexpected admission or clinic visiting, daytime function and QoL.