A natural gas liquefaction process in which natural gas is primarily cooled in a first heat exchange part and secondarily cooled in a second heat exchange part distinguished from the first heat exchange part by using a single closed-loop refrigeration cycle using a mixed refrigerant, the closed-loop refrigeration cycle comprising: a condensation step of partially condensing the mixed refrigerant; a first separation step of separating the mixed refrigerant into a first stream having a liquid phase and a second stream having a gas phase after the condensation step; a first introduction step of introducing the first stream into the first heat exchange part after the first separation step; a first expansion step of expanding the first stream discharged from the first heat exchange part after the first introduction step; a first cooling step of introducing the first stream again into the first heat exchange part to cool the natural gas in the first heat exchange part through the first stream after the first expansion step; a first recovery step of recovering the first stream from the first heat exchange part after the first cooling step; a second introduction step of introducing the second stream into the first heat exchange part after the first separation step; a third introduction step of introducing the second stream discharged from the first heat exchange part into the second heat exchange part after the second introduction step; a second expansion step of expanding the second stream discharged from the second heat exchange part after the third introduction step; a second cooling step of introducing the second stream again into the second heat exchange part to cool the natural gas in the second heat exchange part through the second stream after the second expansion step; and a second recovery step of recovering the second stream from the second heat exchange part after the second cooling step, wherein the first stream is transferred for the condensation step after the first recovery step, and the second stream is transferred for the condensation step after the second recovery step; characterized in that the condensation step comprises: a first compression step of compressing the second stream; a second compression step of compressing the first stream; a third compression step of compressing the second stream after the first compression step; a first mixing step of mixing the first stream with the second stream to form a main stream after the second and third compression steps; a second separation step of separating the main stream into a third stream having a liquid phase and a fourth stream having a gas phase after the first mixing step; a second mixing step of mixing the third stream with the second stream after the second separation step; and a fourth compression step of compressing the fourth stream after the second separation step, wherein the third stream is transferred for the third compression step together with the second stream after the second mixing step, and the fourth stream is transferred for the first separation step after the fourth compression step. a first compression step of compressing the second stream; a second compression step of compressing the first stream; a third compression step of compressing the second stream after the first compression step; a first mixing step of mixing the first stream with the second stream to form a main stream after the second and third compression steps; a second separation step of separating the main stream into a third stream having a liquid phase and a fourth stream having a gas phase after the first mixing step; a second mixing step of mixing the third stream with the second stream after the second separation step; and a fourth compression step of compressing the fourth stream after the second separation step, wherein the third stream is transferred for the third compression step together with the second stream after the second mixing step, and the fourth stream is transferred for the first separation step after the fourth compression step.