Despite the biological relevance, the physical origin of attraction between highly negatively charged DNA strands in condensation remains an open question. We have used microcantilever sensors to study the forces involved in DNA condensation by spermidine. The experiments were performed under flow conditions with gold-coated cantilevers sensitized with thiolated single stranded DNA. The experiments show that above a critical concentration of spermidine, the DNA strands abruptly experiences large attractive forces. The critical spermidine concentration for the transition increases with the monovalent salt concentration. The experiments provide a direct insight of the forces responsible of condensation.