dec2bin?

I just want to ask if there is any function that can convert decimal
number to binary vector.

What do you mean by a binary vector?  Function intToBits might be what you 
are looking for, e.g.

x <- as.integer(1234)
y <- intToBits(x)
options(width=50)
y

[1] 00 01 00 00 01 00 01 01 00 00 01 00 00 00 00
[16] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[31] 00 00

z <- paste((0:1)[1+ (rev(y) == 1)], collapse="")
z

[1] "00000000000000000000010011010010"

gsub("^0*", "", z)

[1] "10011010010"

Do any of these give what you are looking for?

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595