A slide rail assembly, comprising: a first rail having a first longitudinal body, and a pair of first arms extended from two sides of the first longitudinal body, a first passage being defined by the pair of first arms and the first longitudinal body, wherein a first ball raceway is formed on each of the first arms and located within the first passage; a second rail movable within the first passage of the first rail, the second rail having a second longitudinal body, and a pair of second arms extended from two sides of the second longitudinal body, a second passage being defined by the pair of second arms and the second longitudinal body, wherein each of the second arms has a first arm section and a second arm section, a second inner ball raceway is formed on the first arm section, a second outer ball raceway is formed on the second arm section; and a third rail movable within the second passage of the second rail, the third rail having a third longitudinal body, and a pair of longitudinal supporting structures extended from two sides of the third longitudinal body, each of the supporting structures comprising a first supporting arm extended from the third longitudinal body and a second supporting arm bent relative to the first supporting arm, wherein each of the supporting structures of the third rail has a substantially U-shaped cross section, a third ball raceway is formed on the first supporting arm with an arc shape to face toward the second inner ball raceway of the second rail; characterised in that the second arm further has a third arm section extended and bent from the second arm section; wherein the third arm section is extended toward the first arm section and has at least one portion contacting the first arm section. a first rail having a first longitudinal body, and a pair of first arms extended from two sides of the first longitudinal body, a first passage being defined by the pair of first arms and the first longitudinal body, wherein a first ball raceway is formed on each of the first arms and located within the first passage; a second rail movable within the first passage of the first rail, the second rail having a second longitudinal body, and a pair of second arms extended from two sides of the second longitudinal body, a second passage being defined by the pair of second arms and the second longitudinal body, wherein each of the second arms has a first arm section and a second arm section, a second inner ball raceway is formed on the first arm section, a second outer ball raceway is formed on the second arm section; and a third rail movable within the second passage of the second rail, the third rail having a third longitudinal body, and a pair of longitudinal supporting structures extended from two sides of the third longitudinal body, each of the supporting structures comprising a first supporting arm extended from the third longitudinal body and a second supporting arm bent relative to the first supporting arm, wherein each of the supporting structures of the third rail has a substantially U-shaped cross section, a third ball raceway is formed on the first supporting arm with an arc shape to face toward the second inner ball raceway of the second rail; characterised in that the second arm further has a third arm section extended and bent from the second arm section; wherein the third arm section is extended toward the first arm section and has at least one portion contacting the first arm section.