  The equation of state (EoS) of dark energy plays an important role in the
evolution of the universe and arouses great interests in recent years. With the
progress on observational technique, precise constraint on the EoS of dark
energy becomes possible. In this paper, we reconstruct the EoS of dark energy
and cosmic expansion using Gaussian processes (GP) from the most up-to-date
Pantheon compilation of type Ia supernovae (SNe Ia), which consists of 1048
finely calibrated SNe Ia. The reconstructed EoS of dark energy has large
uncertainty due to its dependence on the second order derivative of the
construction. Adding the direct measurements of Hubble parameters $H(z)$ as an
additional constraint on the first order derivative can partially reduce the
uncertainty, but is still not precise enough to distinguish between evolving
and constant dark energy. Besides, the results heavily rely on the prior of
Hubble constant $H_0$. The $H_0$ value inferred from SNe+$H(z)$ without prior
is $H_0=70.5\pm 0.5~{\textrm{km}~\textrm{s}^{-1}~\textrm{Mpc}^{-1}}$. Moreover,
the matter density $\Omega_M$ has an unnegligible effect on the reconstruction
of dark energy. Therefore, more accurate determinations on $H_0$ and $\Omega_M$
are needed to tightly constrain the EoS of dark energy.
