Understanding the complex pore network and fractures are crucial to efficiently producing tight gas and oil reservoirs. A better understanding of gas flooding recovery mechanisms will lead to improved successes of EOR (enhanced-oil-recovery) practices in tight oil formations. In this paper, the gas huff-n-puff efficiency performance in tight oil formations is under surveillance by NMR technology. It is of our interest to estimate the recoverable or movable oil saturation by waterflooding and gas flooding at different types of pore sizes. Fortunately, NMR measurements provide an avenue for calculating the recoverable reserves in different types of pore system (micropores, mesopores and macropores). The NMR T-2 relaxation time closely correlates with the pore sizes. The NMR technique was used to analyze the mechanisms of gas flooding and waterflooding in shale formations from a microscopic scale view. In this paper, a series of nitrogen huff-n-puff experiments were conducted on tight cores and NMR was used in the whole huff-n-puff process to observe the gas flooding efficiency at different cycles. The NMR relaxation spectrum reveals that most of the oil production happened in the first few cycles, less oil is recovered in the subsequent cycles. The recoverable oil of this field falls into a range of 1-100 ms T-2 relaxation pore size system. Oil production only occurs in certain type of pores. Due to the nanometer or micrometer scales of the pores and pore throats, cycle depletion time has considerable effect on oil recovery from tight oil reservoirs in the first few cycles. The literature lacks a study of the NMR investigation of gas huff-n-puff effect in tight formations. The purpose of this current study is to illustrate the application of NMR technique in interpreting the effect of cyclic nitrogen injection in tight oil reservoirs.