It's a cliche, but honesty is the best policy. I went through something similar last month. I'd been with the company for 8 months though. Contract was up, I asked if there was any way they could keep me on, they came back to me with an offer. Literally the day after I accepted their offer... I was selected for a position I thought I was way underqualified for with a huge company ($6 billion quartarly profit in the quarter before they hired me... hmmm, let me think about this for a second... YES). I went there in person and told my old boss and he was fine with it and wished me well. Told me he still wanted me to come to the Christmas dinner even though I wasn't working there anymore, and I still got a Christmas bonus. In my mind I imagined being berated and given a guilt trip or something, but it turned out well. Now, I don't know if your boss will be as accepting... but I'd at least tell them. Even if they're not happy about it, it's not like you'll be using them for a reference since you hardly worked there. Out of respect for them alone, I'd tell them as soon as possible.