The distribution of nanodiamond inside a nanomatrix, which is related to the mechanical and viscoelastic properties, is investigated for a natural rubbernanodiamond composite. The composite is prepared by reacting nanodiamond with deproteinized natural rubber (NRND) in the presence of a tert-butylhydroperoxide (TBHPO)/tetraethylenepentamine (TEPA) radical initiator at 30 degrees C in the latex stage and subsequent drying. The morphology of the composite is observed by three-dimensional transmission electron microscopy. NRND prepared with an initiator exhibits a nanomatrix structure, whereas NRND prepared without an initiator displays an island matrix structure. The nanomatrix is densely loaded with 15 nm or smaller-sized nanodiamond. Both the mechanical and viscoelastic properties of NRND depend upon the morphology. The stress at break and the plateau modulus are 12 MPa and 1.19 x 106 Pa, respectively, when NRND is prepared with a TBHPO/TEPA initiator and contains 25 w/w % nanodiamond, which are 4 and 8 times higher than those of deproteinized natural rubber, respectively.