As Martin noted, use the caption package. It has options to change the formatting of the caption. To left align the caption for all tables use the package option singlelinecheck=off. I've used data from the example in your other question to construct the following example:
\documentclass{article}
\usepackage{longtable}
\usepackage[singlelinecheck=off]{caption}
\begin{document}
\begin{longtable}{|c|c|c|c|c|c|c|c|c|}
\caption{Wyniki rozpoznawania obrazów dla obrazu zapytania 004\_0001.jp w katalogu baseballbat}\\\hline
054\_0020.jpg & 209 & ring &054\_0020.jpg & 209 & 10089.1 & celu & time & 0.98 \\ \hline
\end{longtable}
\end{document}
longtablecenters the caption by default by using\multicolumn{<num>}{c}{...}internally. This seems to be hardcoded. But the manual states that you can use thecaptionpackage to change this. – Martin Scharrer Jun 28 '11 at 18:47\usepackage[singlelinecheck=off]{caption}works for me. – Martin Scharrer Jun 28 '11 at 19:01