  The energy dissipation mechanism in blazar jet is unknown. Blazar's flares
could provide insights on this problem. Here we report statistical results of
XMM-Newton X-ray flares of Mrk 421. We analyze all public XMM-Newton X-ray
observations for Mrk 421, and construct the light curves. Through fitting light
curves, we obtain the parameters of flare-profiles, such as peak flux ($F_{\rm
p}$) and flaring time duration ($T_{\rm fl}$). It is found that both the
distributions of $F_{\rm p}$ and $T_{\rm fl}$ obey a power-law form, with the
same index of $\alpha_{\rm F}=\alpha_{\rm T}\approx1$. The statistical
properties are consistent with the predictions by a self-organized criticality
(SOC) system with energy dissipation in one-dimensional space. This is similar
to solar flare, but with different space dimensions of the energy dissipation
domain. This suggests that X-ray flaers of Mrk 421 are possibly driven by a
magnetic reconnection mechanism. Moreover, in the analysis, we find that
variability on timescale of $\sim1000\ $s frequently appears. Such rapid
variability indicates a magnetic field of $\geq 2.1\delta_{\rm D}^{-1/3}$ G
($\delta_{\rm D}$ is the Doppler factor) in emission region.
