This paper develops an equilibrium model of proof-of-work cryptocurrencies. Equilibrium behaviour of miners and users are characterized for exogenous blockchain protocol metrics. This paper shows that an equilibrium can be achieved in the long run. High fixed mining reward is the reason of instability in current cryptocurrency designs. The equilibrium model has two main implications. First, decentralization and technological improvement in mining are the drivers of low transaction fees and low mining costs. Second, limited block size and mining difficulty, which is endogenously determined, create an incentive mechanism that achieves the sustainability of cryptocurrency in the long run.