  A graph $G$ is \emph{uniquely k-colorable} if the chromatic number of $G$ is
$k$ and $G$ has only one $k$-coloring up to permutation of the colors. A
uniquely $k$-colorable graph $G$ is edge-critical if $G-e$ is not a uniquely
$k$-colorable graph for any edge $e\in E(G)$. Mel'nikov and Steinberg [L. S.
Mel'nikov, R. Steinberg, One counterexample for two conjectures on three
coloring, Discrete Math. 20 (1977) 203-206] asked to find an exact upper bound
for the number of edges in a edge-critical 3-colorable planar graph with $n$
vertices. In this paper, we give some properties of edge-critical uniquely
3-colorable planar graphs and prove that if $G$ is such a graph with $n(\geq6)$
vertices, then $|E(G)|\leq \frac{5}{2}n-6 $, which improves the upper bound
$\frac{8}{3}n-\frac{17}{3}$ given by Matsumoto [N. Matsumoto, The size of
edge-critical uniquely 3-colorable planar graphs, Electron. J. Combin. 20 (3)
(2013) $\#$P49]. Furthermore, we find some edge-critical 3-colorable planar
graphs which have $n(=10,12, 14)$ vertices and $\frac{5}{2}n-7$ edges.
