in my Mikrotik i have Two LAN and 1 WAN ,i'd like to Make Both LAN in same Subnet as one. in Range 12.12.12.0/24 with DHCP enabled
and for the WAN network is 192.168.100.0/24 , and Enable NAT as Masqurade ,is there a solution to merage the two LAN
here is my design for Network :-

Default configuration of any (at least most) MikroTik routers is a single WAN port and multiple LAN ports ("merged" by a bridge).
Can you please show your configuration (/export hide-sensitive file=anynameyoulike) to have an idea where you currently stand?

Reset the configuration to defaults and it will work as you want.

i did with the bridge and I got a problem with some sites ,I'm still working to fix it
Thanks guys

The connection behind your internet router is PPPoE?

i called the ISP and it was MTU ,then I put this MTU on the ISP router and on the Bridge too then it fixed my problem

thanks guys