Square well chains of 4, 8, and 16 segments with well width *λ*=1.5 were investigated by grand ensemble Monte Carlo simulations. We used an unbiased, complete scaling, *Q*-parameter method, to estimate critical temperatures and densities in the thermodynamic limit, with the help of histogram reweighting technique and finite size scaling theory. We showed that a square well chain with more segments has a higher critical temperature than that with fewer segments. The critical temperatures for different chain lengths are all lower than those reported previously. Critical points obtained in this work are more precise because the complete scaling is totally unbiased. The relationship between critical temperature and chain length is in good agreement with the Flory-Huggins theory. We also estimated that the critical temperature for an infinitely long square well chain is a little higher than previous results.