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 Hb 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.