  The shapes of light normal nuclei and $\Lambda$ hypernuclei are investigated
in the $(\beta, \gamma)$ deformation plane by using a newly developed
constrained relativistic mean field (RMF) model. As examples, the results of
some C, Mg, and Si nuclei are presented and discussed in details. We found that
for normal nuclei the present RMF calculations and previous Skyrme-Hartree-Fock
models predict similar trends of the shape evolution with the neutron number
increasing. But some quantitative aspects from these two approaches, such as
the depth of the minimum and the softness in the $\gamma$ direction, differ a
lot for several nuclei. For $\Lambda$ hypernuclei, in most cases, the addition
of a $\Lambda$ hyperon alters slightly the location of the ground state minimum
towards the direction of smaller $\beta$ and softer $\gamma$ in the potential
energy surface $E \sim (\beta, \gamma)$. There are three exceptions, namely,
$^{13}_\Lambda$C, $^{23}_\Lambda$C, and $^{31}_\Lambda$Si in which the
polarization effect of the additional $\Lambda$ is so strong that the shapes of
these three hypernuclei are drastically different from their corresponding core
nuclei.
