Time Series

I'm using a dataset with unequally spaced time series
and I'd want to know if there is in R some function in
order to calculate the autocorrelation function, because
acf() in stats package cannot calculate it, because I
have many missing data, and data are not equally spaced.

The autocorrelation function is a theoretical quantity: acf() estimates 
it, not calculates it.  (Do read its help page carefully.)

For `unequally spaced time series' you would need to define what you mean
by the autocorrelation function, then find a credible estimator.
(One way forward is to assume you have a sample of a continuous-time 
series, but only one.)

And if so, is it necessary to organize dataset in ts
structure or is it possible to consider it like a simple
vector?

None of the above!

Depends what you want to do with it, but the structures offered by the 
packages tseries, zoo and its seem more suitable.

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