Inside-out patch clamp technique was used to study the properties of calcium-activated potassium channel（BK Ca）of neurosecretory cells dissociated from Medulla Terminalia X-organ(MTXO) of Chinese mitten crab Eriocheir sinensis. Calcium-activated potassium channel was observerd in symmertrical high potassium solutions (200 mmol•L－1)，and the single channel showed rapid-open rectangle waves with different durations and had a conductance of (213.4±11.2) pS. At a series of the membrane potentials (from -80 to +80 mV), the current amplitude and the open state probability of single channel presented obviously voltage-dependent. In addition, the open state probability of single channel also increased in proportion to Ca2+ concentration in the bath solution, indicating that the single channel was obviously sensitive to intracellular free Ca2+ concentration. Furthermore, drug sensitivity testing revealed that tetraethylammonium (TEA, 80 mmol•L－1) could completely block the single channel. The experiments suggested that BKCa channel of neurosecretory cells dissociated from MTXO of Chinese mitten crab Eriocheir sinensis is voltage-dependent with a large conductance and is sensitive to Ca2+ and TEA. Key words: Eriocheir sinensis; medulla terminalia X-organ; neurosecretory cell; calcium-activated potassium channel; patch clamp