documentclass[margin=5mm, tikz]{standalone}
usepackage{tikz}
usetikzlibrary{decorations.markings}
begin{document}

begin{tikzpicture}[>=latex, scale=0.5]
draw (0,0) grid (8,8);

tikzset{mymarks/.style={
->,
decoration={markings,
mark=
between positions 0 and 1 step 0.7071 cm
with {arrow{>}}},
postaction={decorate}
}
}

draw[mymarks,red] (0.5,0.5) -- (7.5,7.5);
draw[mymarks,blue] (3.5,0.5) -- (3.5,5.5);
end{tikzpicture}
end{document}