Monte Carlo simulations were used to identify the microphase morphologies of ABA triblock copolymer melts confined in a cylindrical nanotube. The influences of the volume fraction of mid‐block B (fB), the radius of nanotube (R) and the asymmetry of ABA triblock copolymer chain were discussed in detail. When fB varies, a series of double‐continuous, three‐layer concentric cylinder barrel, porous net, double helixes and the new multiplex structures were observed under different conditions. In addition, the stacked disk, catenoid‐cylinder and multi‐layer concentric cylinder barrel structures occur in turns at changing R. The relation between circular lamellae period L and layer number Nlayer of concentric cylinder barrel with the increase of R was investigated to further explain the put‐off phenomenon of microphase transition of the multi‐layer concentric cylinder barrel structures. As for the increase of the asymmetry of ABA triblock copolymer chain, it was concluded that the short AI segments tend to site at the interface between rich A and B circular lamellae.
