Simply using math mode in latex to generate the SI micro- prefix isn’t correct since this will produce an italic symbol.
One way to properly write micrometer is by using the SIunits package, this may well be included as a package with the latex on your computer. An example latex document to achieve this is demonstrated here:
\documentclass{article}
\usepackage[mediumspace,mediumqspace,Grey,squaren]{SIunits}
\begin{document}
\unit{100}{\micro\meter}
\end{document}
There are many options specified for the package SIunits in the example above, the first two can be used to control the use of white space, Grey and squaren can be specified to avoid conflicts with other packages you may be using.

This method helps me a lot! U r genius!
Thanks.
I think it is possible to do this by another method too, but this is still the one I know and use.
The package documentation at http://ctan.org/tex-archive/macros/latex/contrib/SIunits/SIunits.pdf doesn’t agree with the order of the arguments in your \usepackage statement.
This is what works for me:
\usepackage[cdot,mediumqspace,squaren,Gray]{SIunits}
Andy, you might want to look into siunitx, it’s an evolution from SIunits and much more flexible and powerful.
This way work better:
\usepackage{upgreek}
$\upmu$s
taken here http://www.linuxquestions.org/questions/linux-software-2/inserting-microseconds-symbol-in-latex-747611/
There is also texcomp package which allows \textmu to be written in the text. Also txfonts package which provides $\muup$