The cognitive models of information representation and the capacity of human memory are fundamental research areas in cognitive informatics, which help to reveal the mechanism and potential of the brain. This paper develops the object-attribute-relation (OAR) model for describing information representation and storage in the brain. According to the OAR model, the human memory and knowledge are represented by relations, i.e. connections of synapses between neurons, rather than by the neurons themselves as the traditional container metaphor described. Based on the OAR model, the memory capacity of the human brain is calculated as in the order of 108,432 bits. The determination of the magnitude of human memory capacity is not only theoretically significant in cognitive informatics, but also practically useful to estimate the human potential, as well as the gap between the natural and machine intelligence.