The transient grating (TG) method has been applied to the determination of the photodissociation quantum yield (φd) of iodine in various fluids at low-, medium-, and high densities. After a nanosecond laser excitation at 532 nm, the thermal grating created by fast processes such as excess energy relaxation, primary geminate recombination, and secondary geminate recombination appears as a fast rise of the TG signal, and the thermal grating created by the nongeminate recombination process appears as a slow rise of the TG signal. By comparison of the relative intensities of these signals, φd has been determined. We have demonstrated that the species grating and the volume grating are negligible in the TG signal in fluids such as argon at 323 K, krypton at 293 K, and liquid solvents at room temperature. Our results of φd in these solvents show density dependence similar to those previously measured by the transient absorption method (Dutoit, J.-C.; et al. J. Chem. Phys. 1983, 78, 1825. Luther, K.; et al. J. Phys. Chem. 1980, 84, 3072.), although our results are larger quantitatively. The diffusion model (Otto, B.; et al. J. Chem. Phys. 1984, 81, 202) works well for interpretation of the density dependence, although modifications of the theory are required to get a quantitative agreement. In xenon, sulfur hexafluoride, carbon dioxide, ethane, and nitrous oxide, which have critical temperatures near the temperature of this experiment (323 or 333 K), we observed a density dependence of φd similar to that in argon, while a significant decrease of φd in the low-density region of carbon dioxide, ethane, and nitrous oxide was reported previously (Zellweger, J. M.; et al. J. Chem. Phys. 1980, 72, 5405. Otto, B.; et al. J. Chem. Phys. 1984, 81, 202.). Discussion on the origin of the difference is presented on the basis of the intermolecular potential estimated by the solubility data.