We have succeeded in synthesizing macroscopically aligned helical polyacetylene (H-PA) by using magnetically oriented chiral nematic liquid crystal (N*-LC) with monodomain structure The N*-LC with monodomain structure was prepared by applying a magnetic field as an external perturbation to a N*-LC with polydomain structure Because of the positive anisotropy of the diamagnetic susceptibility of N*-LC, the helical axis of the N*-LC was oriented perpendicular to the direction of the applied magnetic field The monodomain N*-LC enabled us to synthesize macroscopically aligned H-PA The bundles of fibrils were aligned parallel to the direction of the magnetic field and perpendicular to the axis of the N*-LC It is anticipated that the macroscopically aligned H-PA film might be available for novel organic electromagnetic materials and also for the pi rinsing precursor of aligned helical graphite by using the morphology-retaining carbonization method