The integration of lanthanide (Ln) ions and polyoxoniobates (PONbs) is challenging, and the known Ln-substituted PONbs are still scarce. This work introduces high-nuclear iso-Ln-oxo clusters into the PONb system. The first series of high-nuclear Ln-oxo clusters encapsulated heterometallic polyoxoniobates H₉[Na(H₂O)₄][Cu(en)₂]₁₀{Ln₆(μ₃-OH)₆(SiNb₁₈O₅₄)₃}∙18H₂O (1-Ln, en = ethylenediamine, Ln = Dy, Gd, Tb, Ho, Er, Tm, Yb, Lu) based on flower-like {Ln₆(µ₃-OH)₆(SiNb₁₈O₅₄)₃} ({Ln₆Si₃Nb₅₄}) clusters have been successfully synthesized via one-pot hydrothermal synthesis strategy. The flower-like polyoxoanion {Ln₆Si₃Nb₅₄} is consisted of three heteropolyoxoniobate {SiNb₁₈O₅₄} clusters and one unique planar equilateral triangle-shaped {Ln₆(μ₃-OH)₆} cluster, which presents the highest nuclear iso-Ln-oxo cluster in PONb chemistry. In {Ln₆(μ₃-OH)₆} cluster, each pair of μ₃-OH groups link three Dy³⁺ ions to form a small approximate equilateral triangle-shaped {Dy₃(OH)₂} cluster. Furthermore, the three {Dy₃(OH)₂} clusters comprise a bigger approximate equilateral triangle-shaped {Dy₆(μ₃-OH)₆} cluster. The reported hexanuclear {Ln₆} cluster skeletons are mostly octahedral, however, such equilateral triangle-shaped skeleton of the hexanuclear Ln-oxo cluster is first observed. The 1-Dy exhibits good water vapor adsorption capacity and ferromagnetic properties.