For the first time poly(methyl methacrylate) (PMMA) nanocomposites based on a synthetic shear-stiff, mica-like K-fluorohectorite clay were prepared by melt -compounding. Besides stiffness, this new synthetic type of clay offers very high aspect ratios above 600 after exfoliation, whereas a tailored surface modification of the clay yielded a good compatibility to the matrix material. The influence of different clay loadings (0-4 wt.-%) on mechanical behaviour of PMMA/clay nanocomposites were studied for two aspect ratios (approximate to 55 and approximate to 620), set into correlation with morphology and compared to natural montmorillonite (MMT) clay, which has an aspect ratio of approximate to 50. It was found, that the use of these novel nanoplatelets leads to a significantly increased fracture toughness of about 25 and 70% in the case of an aspect ratio of 55 and 620, respectively, in comparison to neat PMMA, without sacrificing tensile strength. Scanning electron microscopy (SEM) analysis of the corresponding fracture surfaces and mu-computer tomography (mu-CT) revealed a high dispersion quality of the synthetic organo-clay in PMMA. Different fracture mechanisms could be identified. The presence of the nanofiller varies the local stress state in the matrix and promotes additional energy dissipating mechanisms like crack deflection, crack pinning as well as debonding effects with platelets pull-out leading to enhanced fracture toughness. (C) 2012 Elsevier Ltd. All rights reserved.