After shale gas，the tight oil becomes another new target for the exploration and development of unconventional oil and gas all over the world，and it is paid more attention by domestic petroleum industry. The controversy，however，has been going on throughout the nation over the concept of the tight oil. The paper aims at defining tight oil of the Yanchang Formation in Ordos Basin，based on analysis of the tight oil in the Yanchang Formation from different perspectives including reservoir physical property and mobile fluid. The permeability boundary was determined combining with the exploration and development practice in Ordos Basin to make a discussion on the standard of definition of tight oil. The result shows that conditions for hydrocarbon reservoiring in the Yanchang Formation are good in Ordos Basin. The intervals of Chang6，Chang7，Chang8 and Chang9 are the main areas for tight oil accumulation. The central area and the south of deep lacustrine of the Yanchang Formation are most favorable for tight oil exploration in Ordos Basin. Six parameters were proposed to define the permeability boundary of tight oil based on logging，physical property，movable fluid，radius of main flow throat，threshold pressure gradient，oil viscosity，pressure coefficient and data of oil test. As a result，the classification criteria of reservoir type were modified and the upper limit of reservoir permeability of tight oil was determined as 0.5×10- 3 μm2，which is mainly suitable for tight sandstone oil and shale oil. Therefore，the concept of tight oil should be defined according to geological characteristics，practices of exploration and development in each basin.