John Keats was a remarkable representative of the 19^th century romantic poetry in Britain. Negative capability is a unique aesthetic theory put forward by John Keats, which greatly influenced his poetical writing, and has much room for critical interpretation. This thesis mainly discusses the background and the formative factors of the theory, and then by taking John Keats' well-known poem Ode to a Nightingale as a sample, analyses the relationship between the theory and the imagery, and finally points out the embodiment of the theory in the poem.