Brownian molecular dynamics simulations are carried out on the self-assembly behavior of rod−coil diblock copolymers. The copolymer molecule is represented by a linear chain consisting of definite beads connecting by harmonic bond stretching potential. The rigidity of the rod block is introduced by harmonic potential for bend at a substantially zero bond angle. The micelle structures formed by such copolymers and molecular packing of rod blocks are investigated. Transitions of aggregate structure are found with changing Lennard-Jones (LJ) interaction εRR of rod pairs. The rod blocks tend to align orientationally and pack hexagonally in the core to form a smectic-like disk structure at the higher εRR. With decreasing εRR, a disk micelle is gradually changed to a new string structure, where the twisting of rod blocks packing in the core has been discovered and further breaks into some small aggregates until unimers. The radius of gyration and order parameter of rod blocks are calculated to confirm such a transition from disk to string structure. The regions of thermodynamic stability of disk, string, and small aggregates are constructed in the diagrams of block chain length against εRR and temperature vs εRR. Increase of the rod block length leads to a more dramatic decrease of the critical micelle interaction (CMI) than decrease of the coil block length does. The onsets of string and disk formation move to higher εRR with decreasing rod block length and/or increasing coil length. Meanwhile, the regions of string micelle and small aggregates become wider. Some simulation results are in agreement with existing experimental observations and theoretical predictions.