This study investigated the 3D microarchitecture of cancellous and cortical bones of the femoral neck in rheumatoid arthritis (RA), osteoarthritis (OA) and donor controls. 26 femoral necks (including heads) were harvested during total hip replacement surgeries in 11 patients with RA (mean age 66.7 ± 12.8 years) and 15 patients with OA (67.3 ± 8.4 years). Femoral heads/necks were also harvested from 8 donors (74.9 ± 10.2 years). Bone samples of 10 mm thickness were prepared from each femoral neck and scanned with micro-CT to evaluate microarchitectural parameters. The RA and OA samples showed no significant differences in microarchitectural parameters in cancellous or cortical bone. Compared with the donor controls, bone volume fraction in RA and OA cancellous bone was significantly greater, the structure model index in OA was significantly lower, and the surface density in RA was significantly greater. The RA bone tissues showed erosion and marked osteophyte formation. This study demonstrated that RA and OA have similar trends of overall microarchitectural degeneration in the femoral neck, despite marked erosion in RA bone and osteophyte formation in OA bone. However, we could not eliminate the possibility of local differences between RA and OA bone. The age-related bone loss in RA and OA was less severe than those of normal ageing and osteoporosis, suggesting a compensatory effect of the diseases to increase bone density.