Suitable pH and oxy-reduction conditions in paddy induced by flooding supply optimal conditions for diazotrophs to do N₂ fixation. Paddy biological N₂ fixation makes paddy be different from upland. Paddy can sustain certain productivity under no N fertilization, while upland productivity decreases year by year. Along with an increase of chemical active N application, environment becomes severely polluted. To secure food production and decrease environmental pollution, biological N₂ fixation is urgently to be increased to reduce chemical N fertilizer application. In order to better explore capacities of paddy biological N₂ fixation, we make the reviews on progresses of quantification techniques, controlling factors, enhancement techniques and perspectives in paddy N₂ fixation. We hope this review can offer some helps for further studies on paddy N₂ fixation.