The kinetic pathways connecting the lamellar and gyroid phases in weakly segregated block copolymers are examined by combining the string method and the self-consistent mean-field theory for polymers. In particular, phase transition from arbitrarily oriented lamellae to gyroid is studied. The results reveal that initially the lamella will evolve into a metastable structure via a nucleation and growth mechanism, and then a secondary nucleation of the gyroid occurs in the metastable phase. The observed metastable structures include the perforated lamellae (PL), the O-70 phase, and the tetragonally perforated layers (TPLabc). Nucleation and growth is the dominant mechanism, and prior phase transition passed by HPLabc, which is a long-lived metastable structure, has the minimal energy barrier.