A slide rail assembly , comprising: a first rail comprising a first structure and a second structure ; a second rail movable relative to the first rail ; and a positioning device arranged to the second rail , the positioning device comprising an elastic member and a positioning member , and the positioning member being located at an initial position (S1) in response to an elastic force generated by the elastic member ; wherein the first structure is a hole or a concaved portion, the second structure is adjacent to the first structure , and the second structure comprises an incline or a cambered surface; wherein when the second rail is moved relative to the first rail from a first position (P1) to a second position (P2) in a first direction (D1), the positioning member is moved from the initial position (S1) to an engaging position (S2) through the elastic force released by the elastic member and abuts against the first structure of the first rail ; wherein when the second rail is moved relative to the first rail from the second position (P2) to a third position (P3) in the first direction (D1), the positioning member is moved from the engaging position (S2) to a disengaging position (S3) by guidance of the second structure of the first rail and no longer abuts against the first structure of the first rail ; characterized in that : an operating member is arranged to the first rail , the operating member comprises a first blocking portion , the second rail has a second blocking portion , when the second rail is moved relative to the first rail to the second position (P2), the first blocking portion and the second blocking portion obstruct each other to prevent the second rail from being moved in the first direction (D1). a first rail comprising a first structure and a second structure ; a second rail movable relative to the first rail ; and a positioning device arranged to the second rail , the positioning device comprising an elastic member and a positioning member , and the positioning member being located at an initial position (S1) in response to an elastic force generated by the elastic member ; wherein the first structure is a hole or a concaved portion, the second structure is adjacent to the first structure , and the second structure comprises an incline or a cambered surface; wherein when the second rail is moved relative to the first rail from a first position (P1) to a second position (P2) in a first direction (D1), the positioning member is moved from the initial position (S1) to an engaging position (S2) through the elastic force released by the elastic member and abuts against the first structure of the first rail ; wherein when the second rail is moved relative to the first rail from the second position (P2) to a third position (P3) in the first direction (D1), the positioning member is moved from the engaging position (S2) to a disengaging position (S3) by guidance of the second structure of the first rail and no longer abuts against the first structure of the first rail ; characterized in that : an operating member is arranged to the first rail , the operating member comprises a first blocking portion , the second rail has a second blocking portion , when the second rail is moved relative to the first rail to the second position (P2), the first blocking portion and the second blocking portion obstruct each other to prevent the second rail from being moved in the first direction (D1). <EOS>