We investigate β decays of the neutron-rich nucleus 18N and the structure of the daughter nucleus 18O using the shell model. The reduced transition strengths B(GT) and branching ratios of the β decays in 18N are calculated in the psd and spsdpf shell spaces with the WBT interaction. The calculations in the two different spaces are compared. The psd calculations obtain a better agreement with the observation of the β-delayed neutron emission, which seems to show that the observed properties of 18N and 18O are mainly produced by one particle being excited from the p-shell to the sd-shell.