Among a wide variety of structural categories of transition metal oxides, the spinel, generally expressed by the chemical formula AB2O4, is unique in that an unusually strong geometrical frustration on both “spin” and “charge” channels is anticipated from its pyrochlore B-sublattice. We will overview “simple” spinel oxides with nonmagnetic A ions and transition metal B ions, with emphasis on the consequence of such strong geometrical frustration. The most appealing examples of spin frustration can be seen in CdCr2O4 and ZnCr2O4. In these Cr-spinels, due to the frustration, the degeneracy in the spin excitation is extremely strong, which eventually brings about a spin-driven structural phase transition accompanied with a magnetic ordering. A magnetic field control of the spin frustration will be discussed. [1] In a mixed valent spinel AlV2O4, we find a textbook example of “charge” frustration effect. To suppress the degeneracy in “charge” channel, an unusual charge ordering with “3:1” ratio of valence skipping V2+ and V4+ ions is realized, instead of the ordering of “1:1” ratio of V2+ and V3+ [2] . Interestingly, the first heavy fermion oxide LiV2O4 is located in the close vicinity to these frustrated magnets and charge ordered insulators. From our systematic specific heat and transport studies, a close link between the geometrical frustration and the heavy quasiparticle mass is suggested for LiV2O4. [3] 1. H.Ueda,et al, submitted to PRL 2. K.Mastuno et al, PRL 90, art. no.-096404 ( 2003) 3. C.Urano et al., PRL 85, 1052-1055 (2000) + work done in collaboration with H.Ueda, H. Aruga-Katori, H.Mitamura, T.Goto, K.Matsuno, T.Katsufuji, C.Urano and M.Nohara

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