A simple strategy was explored to systematically control the phase transition of an amphiphilic bottlebrush block copolymer (AmBBCP), poly-[(norbornene-graf t-styrene)-block-(norbornene-graf t-hydroxystyrene)], with polymeric additives, such as poly(ethylene glycol) methyl ether (mPEG), poly(2-vinylpyridine) (P2VP), and poly(methyl methacrylate) (PMMA). The precursor polymers, poly[(norbornene-graf t-styrene)-block-(norbornene-graf t-4-tert-butoxystyrene)], were synthesized by sequential ring-opening metathesis polymerization of ω-endnorbornyl polystyrene and poly(4-tert-butoxystyrene). Acid hydrolysis of the tert-butyl groups in the precursor resulted in the AmBBCP with an ultrahigh molecular weight (∼2880 kDa) and relatively low dispersity (∼1.21). The disordered structures of neat AmBBCP were transformed to ordered lamellae by solvothermal annealing. AmBBCP and mPEG blended well because of H-bonding, maintaining well-ordered lamellae up to 40 wt % mPEG. The phase transition from ordered to disordered state occurred when increasing more than 50 wt %. The AmBBCP blended with P2VP and PMMA was compared. The effect of mPEG on phase transition, domain size, and refractive index and the photonic properties were determined.