In this article, we present a series of YK de novo peptides that promote artificial protein condensates with liquid-like properties within mammalian cells. The YK peptides consist of alternating tyrosine and cationic lysine residues, spanning lengths from 9 to 15 residues. These YK-genetically fused sfGFPs exhibit droplet formation in mammalian cells, and the length of the YK peptide can modulate their dynamic fluidity.