The influence of in-plane fields on the formation of hard bubbles in epitaxial garnet films by using single-pulse bias field was investigated in some detail. A critical in-plane field H0in which is related to the parameters of bubble films, was discovered. When Hin>H0in, formation of hard bubbles no longer occurs. Three physical processes, i.e. the increase, quick decrease and slow decrease of the demarcation field for soft and hard bubble formation H[b] with in-plane fields can be qualitatively interpreted in terms of the effect of in-plane field on stripe domains, as well as the formation of multifingered domains.