For　spark-ignited　gasoline　engine,　cold　start　emission　of　hydrocarbon　(HC)　and　carbon　monoxide　(CO)　is　a　long-term　concern　due　to　a　large　portion　of　HC　and　CO　emitted　during　cold　start　period.　Starting　by　hydrogen　or　adding　a　fraction　of　hydrogen　can　greatly　reduce　the　engine　cold　start　emissions　of　HC　and　CO　due　to　the　carbonless　and　excellent　combustion　characteristics　of　hydrogen.　However,　the　higher　combustion　speed　and　temperature　of　hydrogen　tend　to　cause　more　NOx　emissions.　Therefore　the　amount　of　hydrogen　addition　to　conventional　fuel　should　be　optimized　at　different　engine　operating　conditions.　In　this　paper,　a　spark　ignited　gasoline　engine　was　retrofitted　to　a　hydrogen　gasoline　flexible　fuel　engine　by　adding　a　set　of　hydrogen　injection　system,　and　the　engine　can　be　fueled　with　gasoline,　hydrogen　or　both.　An　electronic　control　system　was　developed　and　applied　to　control　the　injection　timings　and　durations　of　hydrogen　and　gasoline.　In　order　to　reduce　cold　start　emissions　of　HC　and　CO,　the　engine　started　with　hydrogen　and　then　changed　to　run　on　gasoline　with　hydrogen　addition　or　on　pure　gasoline　to　avoid　high　NOx　generation.　The　basic　composition　of　the　control　system　was　introduced　and　the　cold-start　emission　control　strategy　for　the　hydrogen　gasoline　flexible　fuel　engine　was　investigated.　And　finally　the　emission　of　the　vehicle　running　on　the　hybrid　hydrogen　and　gasoline　fuel　mode　was　tested　over　the　New　European　Driving　Cycle　(NEDC),　and　compared　with　that　of　the　original　gasoline　car.　The　experimental　results　showed　that　the　cold　start　emissions　of　HC　and　CO　greatly　decreased　when　the　vehicle　engine　operating　on　hybrid　hydrogen　gasoline　mode.　(C)　2016　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.