Try changing the "mar" setting in a "par" function call?

It's you! Again!Thank you for your reply!

I've tried your method.
It works in a way not like what i want.
Let me rephase my question: I  use layout function to save several(6) plots in pdf device in the papersize of a4;but the margin of paper is huge, kind of waste, so i want to small the paper margin to enlarge each plot.

The parameter mar in this situation smaller each plots' margin which not i want it to and the parameter oma seems to quit working.
But thanks anyway! You are my new hero !

Then you just need to set the "width" and "height" options in the "pdf" call. You can set the size of the plotting area within the device the way you like but in inches. According to my search the size of a DIN A4 paper is 8.3*11.7 inches. So, as an example, if you want to leave half an inch in each of the 4 edges you may use "width = 7.3, height = 10.7". As far as I can tell R will do the centering for you.

Here are some dummy plots I have done ("mfrow" was used instead of "layout" just for simplicity):

And the result was attached.

Many thanks!