This paper reports the multiplexed monitoring of two promising biomarkers, beta-amyloid (Aβ42) and total tau (T-tau), in both buffer and cerebrospinal fluid (CSF) for Alzheimer’s disease (AD) using label-free optical nanosensors. It has been found that 7.8 pg/ml of Aβ42 in buffer and 15.6 pg/ml of T-au in buffer can be readily detected with very good specificity. Based on our measurements, the purchased CSF itself contains Aβ42, whose concentration is estimated to be about 400 pg/ml. Aβ42 and T-tau in the mixtures of Aβ42 and T-tau spiked in CSF have been detected successfully, indicating the feasibility of the optical nanosensors to detect these biomarkers in clinical samples. For the measurements, only a small amount (~1 μl) of the samples is required. This type of sensor is suitable for point-of-care application to diagnose the AD due to its low cost and ease-of-operation.