Bone is one of the preferred places for cancer metastasis, especially for breast, prostate, lung and melanoma cancers. In general, the prognosis for bone metastasis is abysmal with few treatment options available. Some colonies of cancer cells can reside in bone tissue as dormant cells for years before they become aggressive and grow into macrometastases. The interaction between cancer cells and bone tissue during this long period of time is currently not well understood. One technical barrier to study this in vitro is the lack of realistic bone tissue model suit for long-term study. Herein, we reported a bone-on-a-chip microdevice that can spontaneously form of a 3D, mineralized, collagenous bone tissue from an inoculum of isolated osteoblastic cell line without any artificial scaffold materials. Based on the principle of simultaneous-growth-dialysis, phenotypically mature osteoblastic tissue of up to 85 μm thickness containing heavily mineralized collagen fibers naturally formed in 720 hours without the aid of differentiation agents. Moreover, we have examined co-culture of metastatic human breast cancer cells MDA-MB-231 with osteoblastic tissues and observed important hallmarks of breast cancer colonization previously confirmed in vivo. With simple manipulation and ultra-low unit cost, the spontaneous 3D bone-on-a-chip shows promise as a physiologically-relevant model for the in vitro study of cancer bone metastasis.