I am writing a report and use the following preamble:

``````\documentclass[10pt,b5paper,twoside]{report}
\usepackage[lmargin=25mm,rmargin=25mm,tmargin=27mm,bmargin=30mm]{geometry}
``````

Now latex think that the first page is a "left" page, any way I can set the first page to be a "right" page?

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The first page, the one with number 1, is for sure a right page. Perhaps explain why you think differently. – Stefan Kottwitz Jun 14 '11 at 19:14
I agree, LaTeX definitely "thinks" that page 1 is a right page, using your preamble. Perhaps your PDF or DVI viewer thinks differently? Otherwise, what do you mean? – fudo Jun 14 '11 at 20:31
Perhaps if your inner margin were not the same as the outer, you could see a difference? – henrique Jun 15 '11 at 19:41
Adobe Reader by default displays things incorrectly. There is a 'show title page' option (or something like that) somewhere. – Caramdir Aug 5 '11 at 17:28

Not sure what you mean by the "first page", but try it without the `twoside` option:
``````\documentclass[10pt,b5paper]{report}