Yes, there is a way to easily specify a grid line thickness of 0.1 mm. In
Origin 5.0 grid lines are controlled on the Grid Lines tab in the Axis window
(double-click on any axis tick mark to open the Axis window and then click once
on the Grid Lines tab). First, check the Major Grids checkbox and uncheck the
Minor Grids checkbox. Grid line thickness is specified in points where 1 inch
= 72 points but since 1 inch = 2.54 cm you can easily calculate that 0.1 mm =
0.283 points:
2.54 cm = 72 pts
25.4 mm = 72 pts
1 mm = 72/25.4 = 2.83 pts
0.1 mm = 0.283 pts

Enter 0.28 in the Thickness(pts) edit box and you will be all set. This
procedure must be repeated for both the X and Y axes.

You can also specify a grid line spacing of 10 mm. With a graph window active
select Format:Layer from the Origin main menu and specify mm as the Measurement
Units in the Layer 1 Properties window. Then, specify a Width and a Height
that are multiples of 10. Next, open the Axis window again and select the
scale tab. Enter From, To, and Increment values so that a major tick will be
placed every 10 mm (Origin places grid lines at the same interval as tick
marks). For example, I chose a layer width of 200 mm and entered 0 for From,
20 for To, and 1 for Increment in the X Axis Scale tab. You may also want to
specify a Rescale option of Manual so that adding data to the plot will not
force a rescale. These settings will place a major tick and a gridline spaced
every 10 mm. Of course, you must select scale values that insure proper
spacing of the grid lines and that fully display your plotted datasets. This
procedure must be repeated for both the X and Y axes.

You may wish to save your graph window created with the procedures above as a
graph template (File:Save Template As from the Origin main menu) that can
easily be plotted to (Plot:Template) again and again without having to go
through the setup procedure each time.