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We selected a sample of 24 XMM-Newton light curves (LCs) of four high energypeaked blazars, PKS 0548-322, ON 231, 1ES 1426+428 and PKS 2155-304. These datacomprise continuous light curves of 7.67h to 18.97h in length. We searched forpossible quasi-periodic oscillations (QPO) and intra-day variability (IDV)timescales in the LCs of these blazars. We found a likely QPO in one LC of PKS2155-304 which was reported elsewhere (Lachowicz et al. 2009). In the remaining23 LCs we found hints of possible weak QPOs in one LC of each of ON 231 and PKS2155-304, but neither is statistically significant. We found IDV timescalesthat ranged from 15.7 ks to 46.8 ks in 8 LCs. In 13 LCs any variabilitytimescales were longer than the length of the data. Assuming the possible weakQPO periods in the blazars PKS 2155-304 and ON 231 are real and are associatedwith the innermost portions of their accretion disk, we can estimate that theircentral black hole masses exceed 1.2 $\times$ 10$^{7}$ M$_{\odot}$. Emissionmodels for radio-loud active galactic nuclei (AGN) that could explain ourresults are briefly discussed.

Detection of Intra-day Variability Timescales of Four High Energy Peaked Blazars with XMM-Newton