The paper discusses the U(1) problem in the context of 1/N expansion of QCD, addressing the challenges posed by topological charge quantization and the Adler-Bell-Jackiw axial anomaly. The author proposes a modified Kogut-Susskind mechanism to resolve these issues, showing that Ward identities are algebraically saturated for large N. The paper also explores the implications of this mechanism on the θ dependence and the mass formula of pseudoscalar mesons. The results are applied to the physical world, providing insights into the mixing angles and masses of pseudoscalar mesons, and suggesting a possible θ periodicity in large N YM theory. The author acknowledges contributions from several researchers and references relevant literature.The paper discusses the U(1) problem in the context of 1/N expansion of QCD, addressing the challenges posed by topological charge quantization and the Adler-Bell-Jackiw axial anomaly. The author proposes a modified Kogut-Susskind mechanism to resolve these issues, showing that Ward identities are algebraically saturated for large N. The paper also explores the implications of this mechanism on the θ dependence and the mass formula of pseudoscalar mesons. The results are applied to the physical world, providing insights into the mixing angles and masses of pseudoscalar mesons, and suggesting a possible θ periodicity in large N YM theory. The author acknowledges contributions from several researchers and references relevant literature.