Even though nanoliposomal drug delivery systems are superior to conventional drug delivery methods, studies indicate that they may deliver small quantities of drugs through passive targeting. Specific peptides are used to decorate the surface of the liposomes for active targeting of tumors. Among these, CGKRK is the tumor homing peptide to target p32 protein on the surface of endothelial and tumor cells. In the present study, serine-glycine (SG) and glycine-glycine-glycine (GGG) spacer were added to CGKRK, an anti-P32 peptide and in silico molecular docking studies were conducted. PEGylated liposomal doxorubicin (PLD) was then surface modified with these peptides. All liposomal formulations had the sizes between 96-110 nm, negative zeta potentials and there were spherical in shape. The release of the doxorubicin (Dox) followed the same pattern for all formulations during 24 h. The cytotoxic effects of the formulations during 1, 3 and 6 h of the incubation, indicated the time-dependent cytotoxicity of formulations, while the PLD-CGGGKRK was the most potent. Apoptosis and antiangiogenic assays were also demonstrated the ability of PLD-CGGGKRK in inducing apoptosis and decreasing vessels densities. The in vivo anti-tumor study demonstrated the efficacy of CGGGKRK surface-modified PLDs in delaying tumor growth and increasing the median survival. Our findings indicated that the GGG spacer could improve the efficacy of anti-P32 peptide, CGKRK, in targeting breast cancer tumor models in mice and merit further investigation.