Cancer marker has been elucidated as a powerful medical tool in disease prediction, diagnosis, and monitoring. The clinical utility of protein biomarker to discriminate health and disease requires the capability to measure extremely low concentration proteins.  The flexible cancer sensor based on layer-by-layer (LbL) self-assembled graphene demonstrates features including ultra high sensitivity and low cost due to graphene material properties in nature, self assembly technique, and polyethylene terephthalate (PET) substrate.  According to the conductance change of self-assembled graphene, the label free and labeled graphene sensors are capable of detecting very low concentrations of prostate specific antigen (PSA) down to 4 fg/ml (0.11 fM) and 0.4 pg/ml (11 fM) respectively, which are three orders of magnitude lower than carbon nanotube (CNT) sensors under the same conditions of design, manufacture, and measurement.