How to open a dialog passing an object as a parameter in primefaces?

You have been very sparing in your question, but assuming you have a button and that button opens a dialog. To pass an object as a parameter, just do this:

<p:commandButton value="Abre Dialog"
                 onclick="PF('dialogWidget').show()"
                 update=":formDialog">
  <f:setPropertyActionListener target="#{nomeBean.objetoReceptor}" value="#{objetoASerPassado}" />
</p:commandButton>