A slide rail assembly, comprising: a first rail comprising a first wall, a second wall and a longitudinal wall connected between the first wall and the second wall, the first wall, the second wall and the longitudinal wall cooperatively defining a channel, the channel having a channel opening; a second rail; and characterized by: a guiding structure disposed adjacent to the channel opening of the channel and for guiding the second rail to be mounted into the channel via the channel opening; wherein the guiding structure comprises a first guiding portion and a second guiding portion, the first guiding portion is disposed adjacent to one of the first wall and the second wall, the second guiding portion is disposed adjacent to the other one of the first wall and the second wall; wherein the guiding structure further comprises a third guiding portion, and the third guiding portion is disposed adjacent to the longitudinal wall, the third guiding portion comprises a first inclined surface, a second inclined surface and a third inclined surface, the first inclined surface corresponds to a side wall of the second rail, the second inclined surface and the third inclined surface are disposed on opposite sides of the first inclined surface, the second inclined surface corresponds to a third wall of the second rail, the third inclined surface corresponds to a fourth wall of the second rail. a first rail comprising a first wall, a second wall and a longitudinal wall connected between the first wall and the second wall, the first wall, the second wall and the longitudinal wall cooperatively defining a channel, the channel having a channel opening; a second rail; and characterized by: a guiding structure disposed adjacent to the channel opening of the channel and for guiding the second rail to be mounted into the channel via the channel opening; wherein the guiding structure comprises a first guiding portion and a second guiding portion, the first guiding portion is disposed adjacent to one of the first wall and the second wall, the second guiding portion is disposed adjacent to the other one of the first wall and the second wall; wherein the guiding structure further comprises a third guiding portion, and the third guiding portion is disposed adjacent to the longitudinal wall, the third guiding portion comprises a first inclined surface, a second inclined surface and a third inclined surface, the first inclined surface corresponds to a side wall of the second rail, the second inclined surface and the third inclined surface are disposed on opposite sides of the first inclined surface, the second inclined surface corresponds to a third wall of the second rail, the third inclined surface corresponds to a fourth wall of the second rail. a guiding structure disposed adjacent to the channel opening of the channel and for guiding the second rail to be mounted into the channel via the channel opening; wherein the guiding structure comprises a first guiding portion and a second guiding portion, the first guiding portion is disposed adjacent to one of the first wall and the second wall, the second guiding portion is disposed adjacent to the other one of the first wall and the second wall; wherein the guiding structure further comprises a third guiding portion, and the third guiding portion is disposed adjacent to the longitudinal wall, the third guiding portion comprises a first inclined surface, a second inclined surface and a third inclined surface, the first inclined surface corresponds to a side wall of the second rail, the second inclined surface and the third inclined surface are disposed on opposite sides of the first inclined surface, the second inclined surface corresponds to a third wall of the second rail, the third inclined surface corresponds to a fourth wall of the second rail.