Gonadotropin-releasing hormone (GnRH) analogs (e.g., triptorelin) are developed to treat hormone-dependent reproductive cancers. However, these analogs lack a significant direct antitumor activity to make them suitable for hormone-refractory reproductive cancers. In this study, we examined different biological properties of lipid-modified GnRH analogs, with/without D-amino acid substitution at position 6 in prostate and ovarian cancer cells. We revealed that the improved metabolic stability due to lipid-modification and D-amino acid substitution played a pivotal role in enhancing GnRH receptor-mediated direct antiproliferative activity up to 4.5-fold higher than triptorelin. Furthermore, a comparable FSH release and higher LH release activity in pituitary cells than triptorelin was observed, indicating an improved specificity and/or binding affinity to GnRH receptors. We confirmed the important role of sex steroids in the antitumor activity of the lipopeptides, which were contrasting in prostate and ovarian cancer cells. The superior activity of these GnRH analogs over commercial peptides renders promises for developing new GnRH receptor ligands to treat hormone-dependent and -refractory cancers, as well as emerging new targeting moieties for the delivery of anticancer agents in GnRH receptor-overexpressing cancers.