After the discovery of the Higgs boson, physicists keep searching for New Physics in cosmic, energy and intensity frontiers. In the intensity frontier, a key stone is to provide precise Standard Model prediction by controlling the uncertainty from low energy QCD.

Lattice QCD has been developed to tackle the nonperturbative problems in low energy QCD. It has been demonstrated to be a powerful tool in dealing with the ''gold-plated'' observables such as the hadron spectra and decay constants. To meet the requirement of future experiments in the intensity frontier, we need to study more complicated system beyond gold-plated.

Rare kaon decay is such an example. The cleanliness of the Standard Model predication of the branching ratio for the rare kaon decays K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar make them an ideal place to search for New Physics. The CERN NA62 experiment aims for an observation of O(100) events of K^+ -> pi^+ nu nu-bar decay and a 10%-precision measurement of the branching ratio. The KOTO experiment at J-PARC in Japan aims for first observation of the CP-violating decay K_L -> pi^0 nu nu-bar.

In this talk I will present an exploratory study of the long-distance contributions to the K+ -> pi+ nu nu-bar decay amplitude from first principles using lattice QCD. Such study is necessary to interpret future experiments at a 10% precision or better.