Changing tick labels to percent

You can use PercentForm to express tick labels in percents:

ticks = {#, ToString[PercentForm[#]]} & /@ Range[0, 1, .1];

Plot[x, {x, 0, 1}, Ticks -> {Automatic, ticks }]

Alternatively, you can use Quantity[x, "Percent"] to get x%:

ticks2 = {#, ToString[Quantity[100 #, "Percent"], StandardForm]} & /@ Range[0, 1, 1/10];

Plot[x, {x, 0, 1}, Ticks -> {Automatic, ticks2}]

same picture

Update: To label only major ticks:

tickF = Charting`ScaledTicks["Linear"][##] /.
  {a_, _NumberForm, b___} :> {a, ToString @ PercentForm[a], b} &;

Plot[x, {x, 0, 1}, Ticks -> {Automatic,tickF}]

You can add the option

Ticks -> {Automatic, Transpose@{Range[0., 1., 0.1], ToString[#] <> "%" & /@ Range[0, 100, 10]}}

For example

Plot[x, {x, 0, 1}, Ticks -> {Automatic, Transpose@{Range[0., 1., 0.1], ToString[#] <> "%" & /@ Range[0, 100, 10]}}]

Or for a more general range of data we can use a function in Ticks

percentticks[ymin_, ymax_] := Transpose@{Range[ymin, ymax, (ymax - ymin)/10.], ToString[#] <> "%" & /@ Range[0, 100, 10]}

Plot[x, {x, 0, 10}, Ticks -> {Automatic, percentticks}]