In situ femoral preparation refers to implanting a femoral component before the femoral neck osteotomy and without dislocating the hip joint, which allows the implanted femoral component to be used to measure leg length and offset. One hundred hip arthroplasty surgeries among 93 patients were compared with a control group of 15 patients. A modular neck femoral component was implanted in a technique similar to implanting a femoral nail. The differences between the in situ measurements and the preoperative and postoperative radiograph measurements averaged −0.1 mm for leg length (r = 0.89) and −0.37 mm for offset (r = 0.57). In situ leg length measurement allows accurate measurement of leg length and offset and guides surgeons in selecting appropriate modular components to attain a near anatomical hip arthroplasty.