Small amplitude oscillatory shear (SAOS) technique is a valuable and non-destructive test that can be employed to investigate the changes in the structure of fresh cement paste at early age. The test requires the paste to be deformed at very small strain amplitude, i.e. within the linear viscoelastic region. In this paper, the critical strain of various cement pastes was studied, such strain corresponding to the cutoff point between the linear and non-linear regions of the strain sweep. The frequency used in SAOS test was also evaluated. Results showed that the critical strain of the cement paste made without superplasticizer was on the order of 10-5, and the addition of superplasticizer increased the critical strain to about 10-4. The critical strain of cement paste decreased with testing time. The storage modulus (G') measured by oscillatory time sweep at the strain amplitude of 10-5 was very close to that of 10-6, but much higher than that of 10-4. The G' and the loss modulus (G") of cement pastes were shown to continuously increase with testing time. An abrupt drop of G' (or crossover point of G' and G") was observed for the pastes measured by oscillatory time sweep at the strain amplitude of 10-5, but was not observed at the strain amplitude of 10-4.