We investigate the gravitational wave spectrum resulted from the cosmological first-order phase transition. We compare two models; one is a scalar field model without gravitation, while the other is a scalar field model with gravitation. Based on the sensitivity curves of the LISA space-based interferometer on the stochastic gravitational-wave background, we compare the difference between the gravitational wave spectra of the former and the latter cases resulted from the bubble collision process. Especially, we calculated the speed of the bubble wall before collision for the two models numerically. We show that the difference between the amplitudes of those spectra can clearly distinguish between the two models. We expect that the LISA with Signal to Noise Ratio =10 could observe the spectrum as the fast first-order phase transition.
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