By the surface modification strategy, a novel high-nuclearity lanthanide complex with a discrete structure, Er 26 I(μ 3 -OH) 20 (μ 3 -O) 6 (NO 3 ) 9 (IN) 33 (OH) 3 (H 2 O) 33 (noted Er 26 , HIN=isonicotinic acid), has been prepared under hydrothermal conditions. The magnetic properties of Er 26 and two analogue dysprosium assemblies Dy 30 I(μ 3 -OH) 24 (μ 3 -O) 6 (NO 3 ) 9 (IN) 41 (OH) 3 (H 2 O) 38 (noted Dy 26 -I) and Dy 26 I(μ 3 -OH) 20 (μ 3 -O) 6 (NO 3 ) 9 (IN) 31.25 (OH) 4.75 (H 2 O) 41.75 (noted Dy 26 -II) have been studied. The results show that the slow relaxation of the magnetization is newly found in Dy 26 -I constructed from {Dy 26 } and {Dy 4 } cluster units. The {Dy 4 } units in Dy 26 -I, the relative orientation of the complex or the inter-molecular interactions between {Dy 26 } cores might play an important role in the origin of this magnetic behavior.