set different font family for strings in mtext or text?

Thank you very much for the quick reply.

See ?par: check the 'family' paramater.
You can select 'family' for each call to mtext or text.

Yes, I can select 'family' for each call to mtext or text. however, when 
it's necessary to put both Chinese and English in one line, I should 
call text or mtext several times with position explicitly. It will be 
really tedious. The following code have been used for this purpose, 
however, I don't like this design:

put.text <- function(x, y, text, family, font, ...) {
    str.n <- length(text)
    sw.n <- numeric(length = str.n+1)
    sw.n[1] <- 0

    if (missing(family)) family <- rep("", str.n)
    if (missing(font)) font <- rep(1, str.n)

    for (i in 1:str.n) sw.n[i+1] <- strwidth(text[i], family = 
family[i], font = font[i])
    sw <- sum(sw.n)
    for (i in 1:str.n)
       text(x+sum(sw.n[1:i]), y, text[i], family = family[i], font = 
font[i], adj = c(0,0.5), ...)
}

## usage
## plot "??(English)" with different font family
## 'song' is a user defined font family for CJK.
pdf()
plot(1:10, type = "n")
put.text(5, 5, c("??", "(English)"), c("song", "Times"))
dev.off()

However, mixing families is rather ugly, and there are font families
that cover both English and Chinese.

Yes, there are some font families that cover both English and Chinese, 
however, in those font families, the English characters are ugly...

Note that the main problem with postscript() and pdf() is the limited
support in those languages for non-8-bit character encodings: R cannot
magically remove restrictions of languages designed in the 1970s.
See also http://cran.r-project.org/doc/Rnews/Rnews_2006-2.pdf
(referenced from ?pdf)

Well, I have read this paper very careful, so I can draw CJK on the plot 
in postscript() and pdf().

Users of other OSes have the option of using cairographics-based devices
(e.g. cairo_pdf), and so will Windows' users as from 2.14.0 (which is in
RC): however, the font flexibility is far less on Windows.

I will try this device. Thanks for the information.

Regards,
Jinsong