A 12-week, Double-blind, Multicenter, 3-crossover, Placebo Controlled and Randomized Trial to Investigate the Efficacy and Safety of 10mg and 5mg Bambuterol Tablets Once Daily in Patients With Chronic Obstructive Pulmonary Disease (COPD)

Patients with COPD who meet the inclusion criteria will enter the 7-days washout baseline period. After the baseline period, patients will be randomly assigned to one of the following treatment sequences: * Sequence I: bambuterol 10mg (21 days) -washout (7 days) - bambuterol 5mg (21 days) - washout (7 days) - placebo (21 days); * Sequence II: bambuterol 5mg (21 days) -washout (7 days) - placebo (21 days) - washout (7 days) - bambuterol 10mg (21 days) ; * Sequence III: placebo (21 days) -washout (7 days) - bambuterol 10mg (21 days) - washout (7 days) - bambuterol 5mg (21 days). During the treatment period patients will record their adverse events and use of rescue medication (Ipratropium bromide) in a diary. At each visit, pulmonary function tests will be performed. At V2, V4 and V6, forced expiratory volume at one second (FEV1) and forced vital capacity (FVC) are measured at following times: immediately before tablet treatment, and at 0.5, 1, 2, 3, 4, 6, 9, 12hrs after administration of tablets, FEV1 and FVC area under curve (AUC) 0\~12 hours will be analyzed. At V1, V3, V5 and V7, and FVC are measured a time in the morning. Peak expiratory flow rate (PEFR) is measured by Mini-Wright peak flow meter in the morning before treatments. All other data will be evaluated as safety status, and monitoring of adverse events.