The effects of diatomite/oligomers hybrids on the phase morphology and rheology of metallocene-catalyzed linear low-density polyethylene (mLLDPE) were investigated. The interfacial tension between the components of the mLLDPE/hybrids influenced the dispersion of the filler and oligomer in the matrix and thus the ultimate rheological properties. Polyethylene wax (PEW) oligomer had good compatibility with the mLLDPE matrix. When a diatomite/PEW hybrid (HD-b) was added, PEW and diatomite were dispersed separately in the mLLDPE matrix. PEW acted as a plasticizer whereas diatomite acted as a filler in mLLDPE/HD-b. No synergetic effect was observed for HD-b on the viscosity reduction of mLLDPE. Poly(ethylene glycol) (PEG) oligomer was incompatible with mLLDPE but had good affinity to diatomite particles. With the addition of a diatomite/PEG hybrid, a special phase morphology with an encapsulation structure with a rigid core of diatomite and a shell of PEG lubricant formed. This special phase morphology reduced the viscosity of mLLDPE significantly; that is, the addition of diatomite/PEG had a synergetic effect on the viscosity reduction of mLLDPE in comparison with the addition of PEG alone. The effect of the interfacial tension between the components of the mLLDPE/hybrid system on the rheological properties of mLLDPE was investigated. For hybrids to exhibit a synergetic effect on the viscosity reduction of the polymer matrix, they needed to fulfill the following conditions: (1) the fillers had to have good affinity to the oligomer and (2) the oligomer had to be incompatible with the polymer matrix. According to the principles, diatomite was blended with oxidized polyethylene wax (OPEW). This proved that the diatomite/OPEW hybrid exhibited a synergetic effect on the viscosity reduction of polyoxymethylene. (c) 2006 Wiley Periodicals, Inc.