The cyclin-dependent kinases (CDKs) compose a family of serine/threonine kinases that are important in controlling entry into and transition through each phase of the cell cycle. Its cellular activity is tightly regulated through a number of mechanisms: phosphorylation by kinases and dephosphorylation by phosphatases; the binding of specific activating proteins called cyclins; and inhibitors. The synthesis and degradation of the activating cyclins is tightly controlled which drives progression through the cell cycle. Cyclin E associates with CDK2 to drive cells from the G₁ to S phase through phosphorylation of a limited number of targets, including pRb. Structural studies have been carried out for CDK2, CDK7 and CDK4, in complex with cyclins and several chemical inhibitors. It has been reported that G1 entry is regulated in similar way to S phase entry, but involves a different CDK/cyclin complex. The complex CDK3/cyclin C is involved in the regulation of the G1 entry. Furthermore, CDK3 has been shown to be inhibited by Roscovitine, a well-studied CDK2 inhibitor. The present work describes the molecular modeling of the binary complex CDK3-Roscovitine and analyses the structural basis for inhibition of CDK3. We used the crystallographic structure of the complex CDK2-Roscovitine as template. Roscovitine is tightly bound in the ATP-binding pocket of CDK3 and analysis of the intermolecular contacts indicates that the residues of the molecular fork participate in strong hydrogen bonds. The phenyl part of the roscovitine is pointing away from the binding pocket as observed in the crystallographic structure of CDK2-Roscovitine.