Objective: To investigate the viscoelastic properties related to handling characteristics of composite resins.  Methods: A custom designed vertical oscillation rheometer (VOR) was used for rheological measurements of composites. The VOR consists of three parts: (1) a measuring unit, (2) a deformation induction unit that is constructed with a function generator, a power servo amplifier, and an actuator using sound speaker and (3) a force detecting unit using load cell with an oscilloscope for data processing. Two medium viscous composites, Z100 and Z250 and two packable composites, P60 and SureFil were tested. The viscoelastic material function, including complex modulus E^* and phase angle d, were measured via geometric parameters, measured force, and applied strain of the VOR. A dynamic oscillatory test was used to evaluate the storage modulus (E'), loss modulus (E") and loss tangent (tand) of the composites as a function of frequency (企) from 0.1 to 20 Hz at 23^oC. Results: The E' and E" increased with increasing frequency and showed differences in magnitude between brands. The E^*s of composites at 企=1Hz, normalized to that of Z100, were 2.004(Z250), 4.584(P60) and 35.833(SureFil). The magnitudes and patterns of the change of tand of composites with increasing frequency were significantly different between brands. Phase angles d were 7.2^o(SureFil), 26.1^o(P60), 44.1^o(Z250) and 51.6^o(Z100) at 企=1Hz. The relationships between the complex modulus E^*, the phase angle d and the frequency 企 were represented by frequency domain phasor form, E^* (企) = E^*e^id= E^*<d. Conclusions: The viscoelasticity of composites that influences handling characteristics is significant different between brands. The VOR is a relatively simple device for dynamic, mechanical analysis of high viscous dental composites. The locus of frequency domain phasor plots in a complex plane is a valuable method of representing the viscoelastic properties of composites.